Measuring Light Wavelength

TEST: How can we sense light on a particular range of the visible spectrum?
Certainly one of the most accurate sensors for such a thing is the human eye. Equipped with highly refined photosensitive cells the eye is capable of discerning between frequencies which are just a few nanometers apart. That’s why we say we have colour vision.

One of the things I am interested in though is in being able to detect the presence of blue light in an environment. In the last years a lot of work has been put into finding the photoreceptors in the retina that mediate non-visual responses to light. Regulation of circadian rhythms, mood alteration, concentration capacity, all fall under the label of “non-visual responses to light”. It seems like this could be an interesting entry point for our process of experimentation.

The cells that are held responsible for such a task are Intrinsically Photosensitive Retinal Ganglion Cells and the photoreceptor with which they carry that task is Melanopsin. Apparently Melanopsin is sensitive to a light frequency of approximetely 480 nanometers. 480nm correspond to light in the blue range of the visible spectrum as shown in the diagram below.

Visible Spectrum

If these cells can be responsible for such a broad range of “responses” they are probably a good starting point to look into new modes of communication with the sun.

What I am trying to do is to use an LDR to sense blue light. But LDR have a spectral sensitivity very similar to the human eye (since they are build to respond to “visible light”). This means they are more sensitive to frequencies in the middle range of the visible spectrum and less sensitive towards the fringes of it.

All this requires some calculation and experimentation. So the first thing I did was to set up an LDR inside a box and light up a superbright led in front of it. I did this with a blue, green, red and yellow LEDs. Then I repeated the experiment with a blue filter covering the LDR and wrote down the measurements again. The results are:

BLUE LED WITHOUT FILTER: 400 ———— WITH FILTER: 151/180
GREEN LED WITHOUT FILTER: 720 ———— WITH FILTER: 15/20
YELLOW LED WITHOUT FILTER: 710 ———— WITH FILTER: 5
RED LED WITHOUT FILTER: 720/780 ———— WITH FILTER: 5/6

So, blue light gives us lower values from the start (which makes sense if we take into account the LDR’s spectrum sensitivity. And also blue light is the one that is decreased in a smaller percentage when passed through a blue filter.

Percentage of light intensity reduction with blue filter depending on colour light:
Blue light was reduced in: 62.5% – 55%
Green light: 98%
Red light: 91% – 99%
Yellow light: 99%

So clearly the light that was less reduced through the blue filter is blue light as we could expect. Now the second thing we tried is to set up two LDRs and read their values. They were both giving similar values (although never exact). See values below.

Then we covered one of them with the blue filter and tried moving it around different sources of light. This is the setup:

Circuits of LDRs with and without filters

I plugged it into processing to be able to quickly analyse the result as I was moving around the room with my computer and the arduino plugged in. The red line in the graph corresponds to the LDR without filter and the blue light corresponds to the LDR with the blue filter (and therefore more sensitive to blue light). What the graph shows is that the differences between indoors and outdoors are more extreme for the blue light sensor.

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