How to test cutoff frequency of IR filter on camera? Modern cell phones seems to come with IR filters on their cameras. I want to do an experiment to figure out what wavelengths these filters allow to pass and which they block. How would I go about doing this? 
Also, what sort of equipment would I need? I am a university student so I have access to a nicely supplied physics lab. Thanks.
 A: Monochromator rail setup
If you have access to a white light source, a pair of lens (to columnate the light source and focus it into the monochromator), a monochromator and a photodetector, then the experiment is fairy easy.


*

*Measure the spectrum of the white light source, $I_0$

*Place the filter in between the source and the monochromator, $I_f$

*The transmission is given by $T=I_f/I_0$.


It is not required for the experiment to work but if you match the f-number of the lens to the monochromator then you will get optimised signal, http://www.newport.com/Getting-Light-into-a-Monochromator/383722/1033/content.aspx
Optical fibre spectrometer
Alternatively, USB optic fibre spectrometers are fairly common these days, http://www.oceanoptics.com/Products/spectrometers.asp
If you can get one of these then the experiment will be easier. You can probably get away without using lens, if your light source is bright enough. Just point the source directly at the fibre entrance and do two measurements as outlined above. Using the fibre really makes alignment trivial.
Comments
In both measurements its crucial that the intensity of the light source doesn't change, so a thermal source would be best, ideally with a stabilised power supply. In the same spirt the alignment must not change during the experiment because this will also change the intensity collected by the spectrometer.
If you want to work out the optical density spectrum then use
$$OD = \log_{10}\left(\frac{1}{T}\right)$$
It's common to quote filter attenuation in terms of optical density, http://www.thorlabs.co.jp/newgrouppage9.cfm?objectgroup_id=3880
A: Every digital camera has an IR filter on top of its CCD sensor - these sensors are more sensitive to the IR light (700-800 nm) than visible. To get pictures with colors that are close to what we see, IR should be blocked.
The simple way to measure the transmission curve of your filter is to open up your phone and remove the filter from the top of the sensor (like this). Then you would find a spectrophotometer, which is a pretty standard instrument for a lab, put the filter inside (might be tricky for such a small filter) and measure the transmission curve.
If you have a nice phone and don't want to break it, then you can put a light of an incandescent lamp through a monochromator and take a picture at the exit with your phone. As you go up with wavelength, somewhere around 700 nm your images should become much dimmer. But, depending on the brightness of your light source, you will probably see some light all the way up to 1000 nm. Take any TV remote - the LED there emits at 940 nm. Most of the cameras will detect it if you point the remote straight to the lens.
