Interesting point; the &quot;perceptual brightness&quot; formula used in grayscaling calculations is usually this:<p><pre><code>    sqrt(0.299r² + 0.587g² + 0.114b²)
</code></pre>
Real &quot;photosensitizing&quot; chemicals, on the other hand, have spectral response curves, like this one[1] for the silver iodide used in daguerrotypes. (The formula above just being an approximation of the summed spectral response curves of the three cones in the eye.)<p>There&#x27;s a big difference, like you said, but it&#x27;s not an impossible problem; we should be able to create a greyscaling formula for any photographic process with a known response curve. (And honestly, I have no reason to think this hasn&#x27;t already been done in the creation of an Instagram filter at some point.)<p>[1] <a href="https:&#x2F;&#x2F;books.google.ca&#x2F;books?id=9IpaIAcgthQC&amp;lpg=PA119&amp;ots=zEWTsY30T3&amp;dq=silver%20iodide%20photosensitivity%20spectrum&amp;pg=PA120#v=onepage&amp;q=silver%20iodide%20photosensitivity%20spectrum&amp;f=false" rel="nofollow">https:&#x2F;&#x2F;books.google.ca&#x2F;books?id=9IpaIAcgthQC&amp;lpg=PA119&amp;ots=...</a>