Monochromatic Filter Fun – the White Towel Test

Figure 1:  The White towel test on an orange filter

So, I take my walks in parks and on green-ways, and I notice the way people dress.  The women wear bright colors often, and the guys wear the drab and dreary: gray, faded blue, and white.  I guess that’s par for the species for various reasons we won’t delve into, but I had the fleeting thought that these fellows would not look very  much different if they were photographed in black and white, versus color.

I like to think during my walks.  It’s good thinking time, and so these black and white photographs in my head segued eventually into thoughts about monochromatic photography.  Most people think about black and white photography when the word monochromatic is mentioned, but not all monochromatic photography is black and white.  One may argue that black and white photography is *not* monochromatic.

I was thinking about some of my photographic filters, and the effects I had seen them lend to various photographs I had taken.  I have a number of wide bandwidth hobby optics filters that can give an interesting “look” to photos taken under certain conditions.  The green filter, shown in figure 2, has obviously filtered a lot of light out of the beam, seen easily in the “white towel” test:

Figure 2:  Green filter in the white towel test.

Looking at the filter, we see the initial white from the flashlight as the input beam, and we see the green in the output of the filter.  But, where did the other colors go?  Obviously, they were absorbed by the filter.  We know that filters can be reflective or absorptive or both.  The cheap filters, producing wide bandwidths, are generally absorptive only.   The green filter was definitely cheap.

Now let’s look at another filter I have, that is both reflective and absorptive.  We see its “white towel” test in figure 3:

Figure 3:  A absorptive/reflective filter in a white towel test.

In figure 3, I’m “white towel” testing an orange filter that happens to be both reflective and absorptive.  So, it absorbs some colors, and reflects others.  It is somewhat more narrow bandwidth because it uses two different mechanisms together to provide a better result.  Where did the light go?

We see the white light from the flashlight as input to the filter, and we see the orange output (it’s an orange filter).  Many colors must have been absorbed by the filter.  But – whoa – the blue got kicked back!  Looking at figure 1, which is really a different shot of the same orange filter test, we see the reflected blue shooting back past the flashlight body.  So – obviously it is real reflected blue light, and not some mixing artifact.

But – whoa again!  If we look closely at the light in the filter itself, we see: green!  Is the filter accommodating a mixing effect to produce green inside of the filter itself, which is not reflected outside of the glass?  Or is it just light bending occurring at the edge of the filter glass?  Figure 1 shows the green clearly.  Let’s take another look at the green mixing and/or bending in figure 4:

Figure 4Green mixing inside of the filter?  Or light bending?

In figure 4 we can see more clearly some of the blue kickback reflection.  So, is that reflected light going back into the flashlight parabola, then out into the filter again, then back again into the flashlight “dish” repeatedly, producing light concentration?  That’s sorta’ the way a laser works.

OK, so much for white towels.  White towels reflect all colors, which is why white is white, and also why we couldn’t do a black towel test.

So, let’s look at a “monochromatic” shot of my sea shells, taken through the orange filter of figures 1, 3, and 4:

Figure 5:  A shot taken through the orange filter of figs 1, 3, and 4.

It’s not particularly interesting,  but we had fun with a flashlight and an old white towel.  It *is* surprising how much detail can still be had with only a small sliver of the full spectrum of light shining on a subject, or reflected/admitted from it.

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