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Computer Nerd Kev
Guest

Mon Apr 02, 2018 1:45 am   



I've been looking into whether it would be practical to replace the
incandescent bulb in an old 35mm slide projector with a high power
COB LED, such as the Cree CXA3070 series which is about $60 from RS.

http://www.cree.com/led-components/products/xlamp-leds-integrated-arrays/xlamp-cxa3070

The aim is to replace a 500W "DAY/DAK" bulb (or whatever the 240V
equivalent is), as shown here:
http://www.replacementlightbulbs.com/lampdaydak.html
According to that page, the target is therefore around 12,500
Lumens. A little less than the Cree can get to, but two 6,000Lum
Cree LEDs might be an option too.

So far I've pretty much determined that the heatsinking requirements
would prevent any simple "drop-in" replacement. It looks like either
the whole back of the projector would need to be rebuilt, or perhaps
some sort of overhead assembly would carry the LEDs and their large
heatsink, while the light is beamed through the top of the projector
and onto a mirror that directs it down the lens.

I've deemed both these options (as well as water cooling) to be too
much trouble, so I thought I'd ask for any other thoughts.

I'm also unsure of whether the light from the LED would be suitable.

The preferred target for the LED bulb would be an Argus 500 eg.
https://www.ebay.com.au/itm/173239797320

--
__ __
#_ < |\| |< _#

~misfit~
Guest

Mon Apr 02, 2018 7:45 am   



Once upon a time on usenet Computer Nerd Kev wrote:
Quote:
I've been looking into whether it would be practical to replace the
incandescent bulb in an old 35mm slide projector with a high power
COB LED, such as the Cree CXA3070 series which is about $60 from RS.

http://www.cree.com/led-components/products/xlamp-leds-integrated-arrays/xlamp-cxa3070

The aim is to replace a 500W "DAY/DAK" bulb (or whatever the 240V
equivalent is), as shown here:
http://www.replacementlightbulbs.com/lampdaydak.html
According to that page, the target is therefore around 12,500
Lumens. A little less than the Cree can get to, but two 6,000Lum
Cree LEDs might be an option too.

So far I've pretty much determined that the heatsinking requirements
would prevent any simple "drop-in" replacement. It looks like either
the whole back of the projector would need to be rebuilt, or perhaps
some sort of overhead assembly would carry the LEDs and their large
heatsink, while the light is beamed through the top of the projector
and onto a mirror that directs it down the lens.

I've deemed both these options (as well as water cooling) to be too
much trouble, so I thought I'd ask for any other thoughts.

I'm also unsure of whether the light from the LED would be suitable.

The preferred target for the LED bulb would be an Argus 500 eg.
https://www.ebay.com.au/itm/173239797320


Check Aliexpress for car headlight lamps. They might suit and the better
ones (see feedback) have fan cooling.
--
Shaun.

"Humans will have advanced a long, long way when religious belief has a cozy
little classification in the DSM*."
David Melville (in r.a.s.f1)
(*Diagnostic and Statistical Manual of Mental Disorders)

~misfit~
Guest

Mon Apr 02, 2018 7:45 am   



Once upon a time on usenet ~misfit~ wrote:
Quote:
Once upon a time on usenet Computer Nerd Kev wrote:
I've been looking into whether it would be practical to replace the
incandescent bulb in an old 35mm slide projector with a high power
COB LED, such as the Cree CXA3070 series which is about $60 from RS.

http://www.cree.com/led-components/products/xlamp-leds-integrated-arrays/xlamp-cxa3070

The aim is to replace a 500W "DAY/DAK" bulb (or whatever the 240V
equivalent is), as shown here:
http://www.replacementlightbulbs.com/lampdaydak.html
According to that page, the target is therefore around 12,500
Lumens. A little less than the Cree can get to, but two 6,000Lum
Cree LEDs might be an option too.

So far I've pretty much determined that the heatsinking requirements
would prevent any simple "drop-in" replacement. It looks like either
the whole back of the projector would need to be rebuilt, or perhaps
some sort of overhead assembly would carry the LEDs and their large
heatsink, while the light is beamed through the top of the projector
and onto a mirror that directs it down the lens.

I've deemed both these options (as well as water cooling) to be too
much trouble, so I thought I'd ask for any other thoughts.

I'm also unsure of whether the light from the LED would be suitable.

The preferred target for the LED bulb would be an Argus 500 eg.
https://www.ebay.com.au/itm/173239797320

Check Aliexpress for car headlight lamps. They might suit and the
better ones (see feedback) have fan cooling.


Sorry, checked your links and the car lamps probably wouldn't be bright
enough. Still there are some interesting lamps available...
--
Shaun.

"Humans will have advanced a long, long way when religious belief has a cozy
little classification in the DSM*."
David Melville (in r.a.s.f1)
(*Diagnostic and Statistical Manual of Mental Disorders)

Clifford Heath
Guest

Mon Apr 02, 2018 12:45 pm   



On 02/04/18 09:58, Computer Nerd Kev wrote:
Quote:
I've been looking into whether it would be practical to replace the
incandescent bulb in an old 35mm slide projector with a high power
COB LED, such as the Cree CXA3070 series which is about $60 from RS.

http://www.cree.com/led-components/products/xlamp-leds-integrated-arrays/xlamp-cxa3070

The aim is to replace a 500W "DAY/DAK" bulb (or whatever the 240V
equivalent is), as shown here:
http://www.replacementlightbulbs.com/lampdaydak.html
According to that page, the target is therefore around 12,500
Lumens. A little less than the Cree can get to, but two 6,000Lum
Cree LEDs might be an option too.


The total intensity doesn't matter. What matters is how
wide the emitting surface is for the given intensity.

You can't focus the image to less than the subtended arc
of the original source. That's just simple geometry.

Clifford Heath.

Computer Nerd Kev
Guest

Mon Apr 02, 2018 11:45 pm   



Clifford Heath <no.spam_at_please.net> wrote:
Quote:
On 02/04/18 09:58, Computer Nerd Kev wrote:
I've been looking into whether it would be practical to replace the
incandescent bulb in an old 35mm slide projector with a high power
COB LED, such as the Cree CXA3070 series which is about $60 from RS.

http://www.cree.com/led-components/products/xlamp-leds-integrated-arrays/xlamp-cxa3070

The aim is to replace a 500W "DAY/DAK" bulb (or whatever the 240V
equivalent is), as shown here:
http://www.replacementlightbulbs.com/lampdaydak.html
According to that page, the target is therefore around 12,500
Lumens. A little less than the Cree can get to, but two 6,000Lum
Cree LEDs might be an option too.

The total intensity doesn't matter. What matters is how
wide the emitting surface is for the given intensity.


Don't the lenses in the projector take care of this? After
all, the original bulb is only bright in the area of the
filaments. If not, what should I be attempting to match
if not Lumens?

Quote:
You can't focus the image to less than the subtended arc
of the original source. That's just simple geometry.


I'm afraid geometry has always been a weak point for me.

--
__ __
#_ < |\| |< _#

news18
Guest

Tue Apr 03, 2018 12:45 am   



On Mon, 02 Apr 2018 22:36:09 +0000, Computer Nerd Kev wrote:


Quote:
Don't the lenses in the projector take care of this? After all, the
original bulb is only bright in the area of the filaments. If not, what
should I be attempting to match if not Lumens?

You can't focus the image to less than the subtended arc of the
original source. That's just simple geometry.

I'm afraid geometry has always been a weak point for me.


You are not replacing the original buld, but talking about using
something else, which means you also need to duplicate everything you
want to replace/modify.

Frankly, I'm not going to sit and type out all my life education in light
bulds to assist you with your task. You hack to learn, not learn how to
hack.

Clifford Heath
Guest

Tue Apr 03, 2018 1:45 am   



On 03/04/18 08:36, Computer Nerd Kev wrote:
Quote:
Clifford Heath <no.spam_at_please.net> wrote:
On 02/04/18 09:58, Computer Nerd Kev wrote:
I've been looking into whether it would be practical to replace the
incandescent bulb in an old 35mm slide projector with a high power
COB LED, such as the Cree CXA3070 series which is about $60 from RS.

http://www.cree.com/led-components/products/xlamp-leds-integrated-arrays/xlamp-cxa3070

The aim is to replace a 500W "DAY/DAK" bulb (or whatever the 240V
equivalent is), as shown here:
http://www.replacementlightbulbs.com/lampdaydak.html
According to that page, the target is therefore around 12,500
Lumens. A little less than the Cree can get to, but two 6,000Lum
Cree LEDs might be an option too.

The total intensity doesn't matter. What matters is how
wide the emitting surface is for the given intensity.

Don't the lenses in the projector take care of this?


No, a lens cannot focus a beam to a finer angle. If the
original beam is 1 degree wide, a perfectly focussed
lens will produce a 1-degree wide image of the source.

> what should I be attempting to match if not Lumens?

The intensity per square mm. That's why HID lamps are used
in the first place. Every pixel in the image is a miniature
image of the source, so far as is achievable.

Clifford Heath.

Jasen Betts
Guest

Tue Apr 03, 2018 8:45 am   



On 2018-04-02, Clifford Heath <no.spam_at_please.net> wrote:
Quote:
On 03/04/18 08:36, Computer Nerd Kev wrote:
Clifford Heath <no.spam_at_please.net> wrote:
On 02/04/18 09:58, Computer Nerd Kev wrote:
I've been looking into whether it would be practical to replace the
incandescent bulb in an old 35mm slide projector with a high power
COB LED, such as the Cree CXA3070 series which is about $60 from RS.

http://www.cree.com/led-components/products/xlamp-leds-integrated-arrays/xlamp-cxa3070

The aim is to replace a 500W "DAY/DAK" bulb (or whatever the 240V
equivalent is), as shown here:
http://www.replacementlightbulbs.com/lampdaydak.html
According to that page, the target is therefore around 12,500
Lumens. A little less than the Cree can get to, but two 6,000Lum
Cree LEDs might be an option too.

The total intensity doesn't matter. What matters is how
wide the emitting surface is for the given intensity.

Don't the lenses in the projector take care of this?

No, a lens cannot focus a beam to a finer angle. If the
original beam is 1 degree wide, a perfectly focussed
lens will produce a 1-degree wide image of the source.


The projector does not make an image of the light source, but
of the object (slide, flim, DLP or LCD).

The image on the screen approaches the detail-level of the
object. the main qualification for a projector lamp is even
illumination of the object.

--
This email has not been checked by half-arsed antivirus software

Computer Nerd Kev
Guest

Tue Apr 03, 2018 9:45 am   



Clifford Heath <no.spam_at_please.net> wrote:
Quote:
On 03/04/18 08:36, Computer Nerd Kev wrote:
Clifford Heath <no.spam_at_please.net> wrote:
On 02/04/18 09:58, Computer Nerd Kev wrote:
I've been looking into whether it would be practical to replace the
incandescent bulb in an old 35mm slide projector with a high power
COB LED, such as the Cree CXA3070 series which is about $60 from RS.

http://www.cree.com/led-components/products/xlamp-leds-integrated-arrays/xlamp-cxa3070

The aim is to replace a 500W "DAY/DAK" bulb (or whatever the 240V
equivalent is), as shown here:
http://www.replacementlightbulbs.com/lampdaydak.html
According to that page, the target is therefore around 12,500
Lumens. A little less than the Cree can get to, but two 6,000Lum
Cree LEDs might be an option too.

The total intensity doesn't matter. What matters is how
wide the emitting surface is for the given intensity.

Don't the lenses in the projector take care of this?

No, a lens cannot focus a beam to a finer angle. If the
original beam is 1 degree wide, a perfectly focussed
lens will produce a 1-degree wide image of the source.

what should I be attempting to match if not Lumens?

The intensity per square mm. That's why HID lamps are used
in the first place. Every pixel in the image is a miniature
image of the source, so far as is achievable.


I believe you're pointing out that the area over which the
light is emitted has to be similar in order for the total
light output to be relevant. The datasheet for the CXA3070
LED indicates that this area is within a 23mm diameter
circle. The filament arrangement in the original bulb
measures about 10x10mm, but I figured (perhaps
optimistically) that the incandescent bulb would be less
directional than the LED so it would be a good enough
match. Admittedly I'd forgotten about this by the time
I started considering two LEDs, which I'll concede
wouldn't work.

And just in case we're straying off-topic, a reminder
that I'm talking about a 35mm photographic slide projector,
not a digital projector. I don't believe the original
bulb (in the 50+ year old machine) is a HID type.

--
__ __
#_ < |\| |< _#

Clifford Heath
Guest

Tue Apr 03, 2018 11:45 am   



On 03/04/18 17:48, Computer Nerd Kev wrote:
Quote:
Clifford Heath <no.spam_at_please.net> wrote:
On 03/04/18 08:36, Computer Nerd Kev wrote:
Clifford Heath <no.spam_at_please.net> wrote:
On 02/04/18 09:58, Computer Nerd Kev wrote:
I've been looking into whether it would be practical to replace the
incandescent bulb in an old 35mm slide projector with a high power
COB LED, such as the Cree CXA3070 series which is about $60 from RS.

http://www.cree.com/led-components/products/xlamp-leds-integrated-arrays/xlamp-cxa3070

The aim is to replace a 500W "DAY/DAK" bulb (or whatever the 240V
equivalent is), as shown here:
http://www.replacementlightbulbs.com/lampdaydak.html
According to that page, the target is therefore around 12,500
Lumens. A little less than the Cree can get to, but two 6,000Lum
Cree LEDs might be an option too.

The total intensity doesn't matter. What matters is how
wide the emitting surface is for the given intensity.

Don't the lenses in the projector take care of this?

No, a lens cannot focus a beam to a finer angle. If the
original beam is 1 degree wide, a perfectly focussed
lens will produce a 1-degree wide image of the source.

what should I be attempting to match if not Lumens?

The intensity per square mm. That's why HID lamps are used
in the first place. Every pixel in the image is a miniature
image of the source, so far as is achievable.

I believe you're pointing out that the area over which the
light is emitted has to be similar in order for the total
light output to be relevant. The datasheet for the CXA3070
LED indicates that this area is within a 23mm diameter
circle. The filament arrangement in the original bulb
measures about 10x10mm, but I figured (perhaps
optimistically) that the incandescent bulb would be less
directional than the LED so it would be a good enough
match. Admittedly I'd forgotten about this by the time
I started considering two LEDs, which I'll concede
wouldn't work.

And just in case we're straying off-topic, a reminder
that I'm talking about a 35mm photographic slide projector,
not a digital projector. I don't believe the original
bulb (in the 50+ year old machine) is a HID type.


If you put a pin-hole in a piece of paper and hold it up
to the sun, you don't see a pinprick of light on the ground.
You see an image of the sun. The size of the image depends
on the distance; the angle of the light from the two sides
of the sun is about half a degree. If you add a lens in the
hope to produce a pin-prick of light, you'd fail. Every
point on the surface of the lens is receiving light from
all of the sun, and the rays that hit each point stay spread
across the whole half-a-degree no matter what system of
lenses and mirrors you use, because optical systems are
*always* reversible. So the only way to get a beam that's
narrower is to throw away most of the light by using only
the light that passed through *two* pinpricks. That gets
you a beam of light that comes from just one small part
of the sun. Just not a very bright one.

The exact same argument applies to your projector, whether
it's projecting 35mm slides or through an LCD or DLP system.
The wider your original beam angle from the source, the
less resolution (focus) of the projected image. To get a
bright, sharp image, there's no substitute for an emitter
of exceptionally high intensity - a very hot VERY small
spot.

Clifford Heath.

Jasen Betts
Guest

Tue Apr 03, 2018 9:45 pm   



On 2018-04-03, Clifford Heath <no.spam_at_please.net> wrote:
Quote:
On 03/04/18 17:48, Computer Nerd Kev wrote:
Clifford Heath <no.spam_at_please.net> wrote:
On 03/04/18 08:36, Computer Nerd Kev wrote:
Clifford Heath <no.spam_at_please.net> wrote:
On 02/04/18 09:58, Computer Nerd Kev wrote:
I've been looking into whether it would be practical to replace the
incandescent bulb in an old 35mm slide projector with a high power
COB LED, such as the Cree CXA3070 series which is about $60 from RS.

http://www.cree.com/led-components/products/xlamp-leds-integrated-arrays/xlamp-cxa3070

The aim is to replace a 500W "DAY/DAK" bulb (or whatever the 240V
equivalent is), as shown here:
http://www.replacementlightbulbs.com/lampdaydak.html
According to that page, the target is therefore around 12,500
Lumens. A little less than the Cree can get to, but two 6,000Lum
Cree LEDs might be an option too.

The total intensity doesn't matter. What matters is how
wide the emitting surface is for the given intensity.

Don't the lenses in the projector take care of this?

No, a lens cannot focus a beam to a finer angle. If the
original beam is 1 degree wide, a perfectly focussed
lens will produce a 1-degree wide image of the source.

what should I be attempting to match if not Lumens?

The intensity per square mm. That's why HID lamps are used
in the first place. Every pixel in the image is a miniature
image of the source, so far as is achievable.

I believe you're pointing out that the area over which the
light is emitted has to be similar in order for the total
light output to be relevant. The datasheet for the CXA3070
LED indicates that this area is within a 23mm diameter
circle. The filament arrangement in the original bulb
measures about 10x10mm, but I figured (perhaps
optimistically) that the incandescent bulb would be less
directional than the LED so it would be a good enough
match. Admittedly I'd forgotten about this by the time
I started considering two LEDs, which I'll concede
wouldn't work.

And just in case we're straying off-topic, a reminder
that I'm talking about a 35mm photographic slide projector,
not a digital projector. I don't believe the original
bulb (in the 50+ year old machine) is a HID type.


If you put a pin-hole in a piece of paper and hold it up
to the sun, you don't see a pinprick of light on the ground.
You see an image of the sun. The size of the image depends
on the distance; the angle of the light from the two sides
of the sun is about half a degree.


True

Quote:
If you add a lens in the
hope to produce a pin-prick of light, you'd fail.


False

Quote:
Every
point on the surface of the lens is receiving light from
all of the sun,


False (assuming lens much larger than pinhole, and ignoring diffraction)

Just above you said the ground got a image of the sun, which meane
each point on the ground only sees part of the sun. (or some other
part of the sky where dark) the same is true of the lens.
each part of the lens sees all of the pinhole, but only part of the sun.

Quote:
and the rays that hit each point stay spread
across the whole half-a-degree no matter what system of
lenses and mirrors you use, because optical systems are
*always* reversible.


You need to focus the pinhole on the ground if you want to see an
image of the pinhole, (not focus the sun) this is how slide projectors
produce detailed images using emitters 100mm^2 or larger.

--
This email has not been checked by half-arsed antivirus software

Computer Nerd Kev
Guest

Wed Apr 04, 2018 12:45 am   



Clifford Heath <no.spam_at_please.net> wrote:
Quote:
On 03/04/18 17:48, Computer Nerd Kev wrote:
Clifford Heath <no.spam_at_please.net> wrote:
On 03/04/18 08:36, Computer Nerd Kev wrote:
Clifford Heath <no.spam_at_please.net> wrote:
On 02/04/18 09:58, Computer Nerd Kev wrote:
I've been looking into whether it would be practical to replace the
incandescent bulb in an old 35mm slide projector with a high power
COB LED, such as the Cree CXA3070 series which is about $60 from RS.

The total intensity doesn't matter. What matters is how
wide the emitting surface is for the given intensity.

Don't the lenses in the projector take care of this?

No, a lens cannot focus a beam to a finer angle. If the
original beam is 1 degree wide, a perfectly focussed
lens will produce a 1-degree wide image of the source.

what should I be attempting to match if not Lumens?

The intensity per square mm. That's why HID lamps are used
in the first place. Every pixel in the image is a miniature
image of the source, so far as is achievable.

I believe you're pointing out that the area over which the
light is emitted has to be similar in order for the total
light output to be relevant. The datasheet for the CXA3070
LED indicates that this area is within a 23mm diameter
circle. The filament arrangement in the original bulb
measures about 10x10mm, but I figured (perhaps
optimistically) that the incandescent bulb would be less
directional than the LED so it would be a good enough
match. Admittedly I'd forgotten about this by the time
I started considering two LEDs, which I'll concede
wouldn't work.

If you put a pin-hole in a piece of paper and hold it up
to the sun, you don't see a pinprick of light on the ground.
You see an image of the sun. The size of the image depends
on the distance; the angle of the light from the two sides
of the sun is about half a degree. If you add a lens in the
hope to produce a pin-prick of light, you'd fail. Every
point on the surface of the lens is receiving light from
all of the sun, and the rays that hit each point stay spread
across the whole half-a-degree no matter what system of
lenses and mirrors you use, because optical systems are
*always* reversible. So the only way to get a beam that's
narrower is to throw away most of the light by using only
the light that passed through *two* pinpricks. That gets
you a beam of light that comes from just one small part
of the sun. Just not a very bright one.

The exact same argument applies to your projector, whether
it's projecting 35mm slides or through an LCD or DLP system.
The wider your original beam angle from the source, the
less resolution (focus) of the projected image. To get a
bright, sharp image, there's no substitute for an emitter
of exceptionally high intensity - a very hot VERY small
spot.


Between the bulb and the slide there is a diffuser (as well
as a couple of lenses), so the light hitting the slide
isn't an exact image of the source. I've been playing
around with the removable inner lens assembly using some
torches and it would seem that a slightly wider source
would still illuminate the slide well enough.

If a wider beam angle reduces the resolution, then should
I be able to approximate a larger source beam angle by
using a much shorter projection distance and observing
the decrease in image clarity?

--
__ __
#_ < |\| |< _#

Computer Nerd Kev
Guest

Wed Apr 04, 2018 12:45 am   



Jasen Betts <jasen_at_xnet.co.nz> wrote:
Quote:
On 2018-04-03, Clifford Heath <no.spam_at_please.net> wrote:

and the rays that hit each point stay spread
across the whole half-a-degree no matter what system of
lenses and mirrors you use, because optical systems are
*always* reversible.

You need to focus the pinhole on the ground if you want to see an
image of the pinhole, (not focus the sun) this is how slide projectors
produce detailed images using emitters 100mm^2 or larger.


That seems to make more sense to me, for what it's worth.

--
__ __
#_ < |\| |< _#

Clifford Heath
Guest

Wed Apr 04, 2018 3:45 am   



On 04/04/18 06:01, Jasen Betts wrote:
Quote:
On 2018-04-03, Clifford Heath <no.spam_at_please.net> wrote:
On 03/04/18 17:48, Computer Nerd Kev wrote:
Clifford Heath <no.spam_at_please.net> wrote:
On 03/04/18 08:36, Computer Nerd Kev wrote:
Clifford Heath <no.spam_at_please.net> wrote:
On 02/04/18 09:58, Computer Nerd Kev wrote:
I've been looking into whether it would be practical to replace the
incandescent bulb in an old 35mm slide projector with a high power
COB LED, such as the Cree CXA3070 series which is about $60 from RS.

http://www.cree.com/led-components/products/xlamp-leds-integrated-arrays/xlamp-cxa3070

The aim is to replace a 500W "DAY/DAK" bulb (or whatever the 240V
equivalent is), as shown here:
http://www.replacementlightbulbs.com/lampdaydak.html
According to that page, the target is therefore around 12,500
Lumens. A little less than the Cree can get to, but two 6,000Lum
Cree LEDs might be an option too.

The total intensity doesn't matter. What matters is how
wide the emitting surface is for the given intensity.

Don't the lenses in the projector take care of this?

No, a lens cannot focus a beam to a finer angle. If the
original beam is 1 degree wide, a perfectly focussed
lens will produce a 1-degree wide image of the source.

what should I be attempting to match if not Lumens?

The intensity per square mm. That's why HID lamps are used
in the first place. Every pixel in the image is a miniature
image of the source, so far as is achievable.

I believe you're pointing out that the area over which the
light is emitted has to be similar in order for the total
light output to be relevant. The datasheet for the CXA3070
LED indicates that this area is within a 23mm diameter
circle. The filament arrangement in the original bulb
measures about 10x10mm, but I figured (perhaps
optimistically) that the incandescent bulb would be less
directional than the LED so it would be a good enough
match. Admittedly I'd forgotten about this by the time
I started considering two LEDs, which I'll concede
wouldn't work.

And just in case we're straying off-topic, a reminder
that I'm talking about a 35mm photographic slide projector,
not a digital projector. I don't believe the original
bulb (in the 50+ year old machine) is a HID type.


If you put a pin-hole in a piece of paper and hold it up
to the sun, you don't see a pinprick of light on the ground.
You see an image of the sun. The size of the image depends
on the distance; the angle of the light from the two sides
of the sun is about half a degree.

True

If you add a lens in the
hope to produce a pin-prick of light, you'd fail.

False

Every
point on the surface of the lens is receiving light from
all of the sun,

False (assuming lens much larger than pinhole, and ignoring diffraction)

Just above you said the ground got a image of the sun, which meane
each point on the ground only sees part of the sun. (or some other
part of the sky where dark) the same is true of the lens.
each part of the lens sees all of the pinhole, but only part of the sun.

and the rays that hit each point stay spread
across the whole half-a-degree no matter what system of
lenses and mirrors you use, because optical systems are
*always* reversible.

You need to focus the pinhole on the ground if you want to see an
image of the pinhole, (not focus the sun) this is how slide projectors
produce detailed images using emitters 100mm^2 or larger.


I think that only works because the slide is a diffuser,
and the geometry blocks the straight-through light (that
would cause a flare). So maybe an LED would work for the OP.

Jasen Betts
Guest

Wed Apr 04, 2018 8:45 am   



On 2018-04-04, Clifford Heath <no.spam_at_please.net> wrote:
Quote:
On 04/04/18 06:01, Jasen Betts wrote:


Quote:
You need to focus the pinhole on the ground if you want to see an
image of the pinhole, (not focus the sun) this is how slide projectors
produce detailed images using emitters 100mm^2 or larger.


I think that only works because the slide is a diffuser,
and the geometry blocks the straight-through light (that
would cause a flare). So maybe an LED would work for the OP.


there's no need for the slide to be a difuser, and most aren't.

you don't get flare because the optics before the slide ensure that
the object plane is evenly illuminated, with no slide in place the projection screen
will be is evenly illuminated, with a slide in place some of the
screen will be dark.

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