On Tue, Nov 12, 2013 at 12:10 PM, Patrik D'haeseleer <patrikd@gmail.com> wrote:
I'm not sure why people are so obsessed by 470nm specifically. If you look back at the original literature, you'll see that the response is quite broad, and is actually a fairly good match for the overall blue channel response you posted earlier.

I have had this in mind while designing this app. If I thought I needed real precision, I wouldn't have tried at all, because I have some idea of the limits of the setup. However, 470nm is supposed to be the frequency that people react most strongly to, and people who have read popular interest articles on the subject will be familiar with this. 
 

Unfortunately, the Nikon D200 is not the target platform. It's the iPhone 4 and newer models and I'm having a hard time finding spectral response data on the newer models. (AFAIK, they are variants of the IMX145, and I don't know if the IMX145 datasheet would apply.) Looking at the spectral response of other hardware is good enough to validate this *might* work. However, I really should validate the resulting instrument by accurately measuring what its response actually is. "Yeah, it's probably good enough" is ok for my personal use, but it's really a different story if I'm going to distribute it. 
 
So yeah, for marketing purposes, it's probably more effective if you can tap into the hype around 470nm. But if you really want an app that's effective against insomnia, you might as well measure the blue channel all by itself...

As I said before, if I thought I really needed much more accuracy than that, then I would not have proceeded with this. Even so, I really need to be able to tell people precisely what they are getting. What if the peak turned out to be more like 450nm and dropped off unexpectedly quickly? A user would want to know. If I know exactly what the curve is and can provide that information, knowledgeable people like you can make an informed decision about how well this works. 
 
PS: Also, simply subtracting the red channel from the blue gives you such an incredibly crude measure of the light incoming at any one particular frequency,

The intent has always been to create a relatively crude instrument. 
 
that using highly precise measurement equipment to validate the result is rather nonsensical.

Uh, no. Home audio equipment is comparatively crude, and most consumers know this, but equipment of decent precision is still used to measure its performance. This isn't nonsensical. It's providing good information to users. Really, they have enough uncertainty to deal with when making decisions and unlike yourself, they don't necessarily have a lot of experience gauging how inaccuracies can stack in a system. The more of that kind of judgement you can remove from their process, the easier it is for them. 
 
The DIY spectroscope Public Lab really gives you more than enough precision for what you need (although perhaps not enough for what your customers *want*, but what do they know...)

It's what they can *know* not what they can make an educated guess at in a field they know relatively little about. I don't think, you're really putting yourself in the shoes of the target demographic.