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what surface do you use for evaluating lights?
Posted by
coppercone2
on 15 Apr, 2024 02:16
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I have been playing with 740nm LED (it almost qualifies as thermal for this forum) and I was wondering since its the defacto optics forum,
what surface should you use for evaluating the color of light. Do they have special surfaces for this?
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#1 Reply
Posted by
jonpaul
on 15 Apr, 2024 02:33
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Kodak Gray card, white card used in photography
Jon
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so like if you compare two lights this is a good way? I noticed when I look at red and deep red, what I used to associate with being red is now orange lol
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#3 Reply
Posted by
DaJMasta
on 15 Apr, 2024 04:04
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Reflectance standards (or on the cheap side, white balance cards) are available for the optical band, but they do get somewhat trickier to find as you get farther out of band. For 740nm, I'd expect basically any option that's good in visible to be doable, but you're going to want reflectance graphs for your material of choice (and always as matte finish as possible) if it needs to be accurately representing intensity between bands.
If you're trying to capture all the light from your source and accurately pass it to a spectrometer or similar for measurement, look at integrating spheres - not cheap, but basically a matte reflective material all over a spherical interior surface with some mechanism to hold/power a device in the middle.
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#4 Reply
Posted by
Berni
on 15 Apr, 2024 05:33
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You can buy characterized white surfaces from most suppliers that deal with optics. Another option is also a ground glass difusor.
But for something quick and easy, white wall paint will do. They are made of mostly titanium oxide as the white pigment and they dry into a mat finish. Titanium oxide is really good at looking white in the visible range. It is even reflective quite a way into the IR. It just won't work for UV since it suddenly becomes very absorbent in those wavelengths (this is why it is also used in sunscreen)
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how much better are these things then printer paper? I am just interested in eyeballs at the moment.
I have a 800 and 1000nm power LED that you can see too. I think I can setup a experiment looking at the 880nm vs 740nm on the same sheet if I turn the 880 way up and 740 way down. then I can find a 695nm source of some kind and compare that to the red of the 740 and 880 on adjusted dark eyes. 1000nm is visible too if you blare it but just a little bit, I don't think I can get that one to illuminate anything and its probobly going to be at dangerous power levels if it does
its weird because i renember those old camera illuminators glowing kinda purpleish (like early 2000's IR floodlights for camera, i.e. x10) , but the single power diode IR just looks a bit red.. I wonder if its age or what
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#6 Reply
Posted by
Berni
on 15 Apr, 2024 06:32
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Printer paper is pretty good at being white, but as long as you don't have UV involved. It commonly has a dye added to it that fluoresces blue when exposed to UV, this helps the paper look even more white and helps mask yellowing.
Found a research paper that tested printer paper for reflectance:
https://www.researchgate.net/figure/Reflectance-values-of-office-paper-measured-with-R-0-and-without-R-g-fluorescence-The_fig1_47337184Not quite the flattest and you can see the blue fluorescence trick where it goes above 100% for blue. But it does pretty well for IR, other charts show it reasonably flat down up to around 1200nm.
As for IR LED looking purple, that is mostly a problem of bad filters on cameras. The RGB color filters on camera sensors mostly let trough IR light so it looks whiteish or purpleish (they don't all let equal amounts trough it). This is fixed by placing a IR blocking filter in front of the camera sensor, but the filters used >20 years ago were not very good and still let a significant amount of IR pass trough it. Modern cameras now use much better IR filters and likely the composition of the RGB filters on camera sensors also changes, turning the IR light that makes it in into a different tint.
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I almost swear I have a weak memory of it maybe looking purple to the naked eye but I wonder if I remember maybe looking at one with another camera
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#8 Reply
Posted by
Berni
on 15 Apr, 2024 09:21
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Probably remember seeing it trough a camera yeah, i personally don't remember seeing a IR led lighting up purple to my eyes.
Also the plastic black IR pass filters that block visible light (like used for IR windows or filters on IR photodiodes) tends to have a purplish tint to it, so if you put visible light trough those it looks purple too.
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#9 Reply
Posted by
jonpaul
on 15 Apr, 2024 09:53
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Coppercohne:
Yes white paper is fine, use light source that is daylight:
Full sun and open shade, not a LED, lamp, etc.
The gold std for color match is 5600 K daylight.
Eyes may be the ssue: Variations in color preception over time or between the two eyes is common and indicitave of cataracts.
If you are over 50yrs suggest to see and Opthalamist eye doctror for a full exam.
The usual progression is very slow, over many years. first sign is blurring and glare at night and loss of violet /blue colot
In the end stage you habe a yello or brown cast and poor visiion for driving.
After my cateract operation 10yeas ago, a new world of light and color was revealed.
Take case,
Jon
Sugget
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they do say something about PWM IR being visible at 1/2 the frequency, i.e. 1000nm = green.
could I have seen a PWM IR led at 830nm that look like 415 or something like that? maybe 50% duty cycle PWM if that is true about the eye seeing blinky ir
Been meaning to try that. it would require ssome advanced technology like AC wave forms and pressing buttons
i knew it was a good idea to take this twisted pair of wire and put it in a wire storage box with zip ties instead of throwing the 2 foot segment away. now i might try this..just solder it on and plug into audio amp.i think i can do it
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hmm I plugged it into a hp DC power amp and while it blinks at 1hz if I set it up high to like 200Hz or KHz it looks dimmer (less red) but no green for the 1000 or 800
maybe I will hook it up to my new fluidic cooler mini lab system later and try it at a power level harder then what my palm can sink
or maybe they meant low power levels in total darkness to see it. because they say frequency of blinking lets ppl see the light in remote controls or something in strange colors
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#12 Reply
Posted by
IR_Geek
on 15 Apr, 2024 21:04
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...
If you're trying to capture all the light from your source and accurately pass it to a spectrometer or similar for measurement, look at integrating spheres - not cheap, but basically a matte reflective material all over a spherical interior surface with some mechanism to hold/power a device in the middle.
DajMasta has it nailed. That peaks in the NIR band and what you are seeing is the edges of the spectrum. Depending on a persons ability, they will see slightly different coloring. Also there is the photopic (cones) vs scotopic (rods) that our eyes see for color (bright) and b/w (low light). There should be a data sheet showing the full spectrum. An example from Mouser ...
https://www.mouser.com/datasheet/2/723/XLamp_XPE2-3402122.pdf on page 14 there is a chart showing all the spectrums from that lineup of LED's. I included a screenshot from page 14. Note the spread of wavelengths for each LED.
The purple you remember is probably from a camera looking at the LED
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#13 Reply
Posted by
_Wim_
on 19 Apr, 2024 06:07
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...look at integrating spheres - not cheap, but basically a matte reflective material all over a spherical interior surface with some mechanism to hold/power a device in the middle.
Quite possible to DIY also for not that high of a cost, see
Styrofoam hollow spheres that can be bought at a crafts store also work as quite an acceptable sphere. What kind of spectrometer you want to use it with?
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I found video of a 740 nm laser, its red, pretty sure that guy can see it. but alot of manufacturers claim that 750nm is the end of visible range. the laser diode should be really narrow band, but its a little expenisve