We’ve recently acquired a digital spectral analyzer so I thought we could test some of the LEDs and filter materials used on the forum. For most of the LEDs, the optical characteristics are usually specified by the producer but they are not always correct. In other cases the wavelength is slightly different or for homemade filters their optical properties are practically unknown.
First of all, the spectrometer is a USB 4000 VIS-NIR from Ocean Optics that has been previously calibrated. It has less than 1nm measurement error.
Environmental lights LEDs
(Bandpass) 850DF28 from omegabob at 0 angle
(Bandpass) 850DF28 from omegabob at 30 angle (for viewing the shifting effect)
(Longpass) Edmund Optics NT45-070
Negative film 1 layer
Negative film 2 layers
Floppy 1 layer
Floppy 2 layers
Halogen light bulb
Environmental LEDs are not at exactly 850nm CW. It’s around 845. It could be interesting to see a 845BP10 filter in action.
SFH485 and HSDL-4230 have a much wider emission spectrum than ENv. Lights LEDS. This might be useful since the shifting effect moves the central wavelength of a good filter by more than 30nm. For Env. Lights LEDS, a narrower emission might cause problems with wide lenses.
The hot mirror of the PS3 (and most other cameras) is very efficient at what it does- removing IR radiation. I managed to salvage one intact from the lens assembly and after 700nm it’s almost dark.
Negative films either in 1 or 2 layers are longpass filters rather than bandpass. 2 layers are better than one since they cut most of the visible radiation but the intensity of the transmitted radiation decreases.
Floppy disks are extremely poor in visible filtering applications. Not only they decrease the transmission efficiency but they do a lousy job at cutting visible radiation. I included the spectral analysis of 1 and 2 layers but 2 layers are almost opaque.
850DF28 bandpass filter works great as any other professional filter. I also tested the shifting effect by lighting the filter at a 30 degree angle. It’s easily seen how much the center wavelength is modified.
Neon lamps emit very little in infrared. I didn’t have an energy efficient light bulb to test it but I will after the well-deserved leave . I also included a normal tungsten lamp to test the filters and to prove how much infrared radiation is emitted by those lamps.
I tested just what I had lying around. In time I’ll test more and if anyone is unsure about what they have they can send me a sample (either a LED, filter, any other material) and I’ll perform the spectral analysis.
P.S. Since version 2 of the community book will include more info, perhaps it would be a good thing to include some of the data from the spectrometer for home-made filters or LEDs so that the readers can understand more about the physical part of what is happening and for choosing the right solution.
Print screens of the spectral data to follow:
A normal tungsten light bulb. It gives a lot of infrared radiation
A neon light for reference. Almost zero IR output
A LED from Environmental Lights. Not quite 850nm but close enough
SFH485, a common solution for FTIR
HSDL-4230. More expensive than SFH485 but with better spectrum emission and similar optical power (good for a wide filter, less good for a narrow one - wavlength wise)
The hot-mirror filter of the PS3 webcam. Excellent IR cutting
A long-pass filter for reference (Edmund optics NT45-070). It cuts everything below 650 but lets all the NIR sub-spectrum in.
Negative film 1 layer. More of a longpass than a bandpass, still pretty good results.
Negative film 2 layers. Better filtering but less transmission.
Floppy disk 1 layer. Low transmission and bad filtering characteristics. It does nothing to prevent visible light
Floppy disk 2 layers. Almost opaque, not a good option
850DF28. Great filter, a real bandpass solution
850DF28 at 30 degrees angle. The shifting effect so many people have encountered with LLP is clearly seen here