# Light detector linearity with varying wavelength

Given the same intensity of light, in varying wavelengths, which is typically more linear with their output: Cds (photo resistor) or a Photo transistor?

I would assume that the junctions in the transistor or photo diode would be much more sensitive to a smaller range of light frequencies, but I'm not finding any information to back this up.

Edit: I added an answer below from the research I was able to do after the initial answer. Hopefully it is helpful to some.

What you are asking is how flat the spectrum of various light measuring devices is. Good devices of either type will have datasheets that tell you this. Usually there is something called a "spectral sensitivity" graph.

If I remember right, photodiodes have narrower spectral sensitivity than CdS cells in general, but can be made to different center wavelengths. For example, 980nm is a common wavelength for IR detectors.

One way of getting a flat spectrum is to use multiple sensors, each peaking at a different wavelength or with a different filter in front of them. Software then uses the various values to compute the "flat equivalent" intensity.

• Often figuring out that you are looking for is all about knowing the correct term. "Spectral sensitivity" was the special sauce to get Google to return what I've been looking for.
– Joe
Jun 22, 2011 at 22:22
• More or less what I wanted to answer. Damn. :-) Jun 23, 2011 at 0:19
• @stevenvh, gotcha this time. It seems I can't get a good answer in therefore before you until you finally go to bed for the night. Jun 23, 2011 at 12:29

Photo resistors are much less linear than I would have expected. CdS have peaks around 540nm +-50nm (green). Less common Cd(S.Se) peaks around 630nm +-50nm (red) and CdSe peaks around 720nm +-50nm (infrared). The falloff of CdS is around 50% relative sensitivity +-100nm from peak. This is fairly close to human eye sensitivity in falloff.

Most photo transistors are peaking at around 850nm, far into IR range and rapidly falling off in the visual ranges. There are some photo transistors that are replicating the spectral sensitivity of the human eye as a replacement for the CdS cell. (Cadmium is a toxic chemical that ROHS efforts are trying to eliminate.)

The photo transistors I found based in visible wavelengths are flatter for green and red, with similar sharp dropoffs as CdS for blue. They were however, highly sensitive to IR. Many had sensitivity to 1000 nm (high IR) near that of a 430 nm (violet) and the sensitivity drops much slower up in the IR range, rather than the steep drop for the blue area of visual spectrum. For a full replacement of a CdS cell, including IR immunity, an IR blocking filter may be needed.