The portion of the electromagnetic spectrum that is visible to (can be detected by) the human eye is called visible light or simply light. Human eye responds to wavelengths from about 390 to 700 nm.

Visible-spectrum

collodion spectrumCollodion, like Daguerreotype, displays a sensitivity to light limited to wavelengths under 600 nm, but extends also to UV light, invisible by the human eye. The spectrum of collodion is visible in this figure, kindly provided by my friend Niles Lund (who makes wonderful supplies for wet plate collodion): you can say in b&w the image made over collodion in relation to the visible spectrum. Using his own words: the actinic spectrum was generated by a prism and was directly photographed using collodion. A photograph of the same spectrum was taken simultaneously with digital color. The two photographs were then overlaid using registration marks to insure accuracy.

What does happen when we take a shot with collodion? Blue and purple are brilliant, and can appear almost white, or much clearer than they appear to the sight. Collodion is blind to deep yellow and red, that is, they appear black even when they looks brilliant to the eye. However this is true for pure colors of surfaces which absorb incident light without reflection or refraction, that is an abstract thought. Every surface can reflex light (this is why a red fruit can shine and appear texturized and not as a black hole) and actually refract some diffuse light, with a variable amount of other wavelengths. In addition do not confuse spectral or pure colors with composite, unsaturated colors like pink, magenta, or other purple variations, which we call "red" but are actually made by a mix of multiple wavelengths and can result in bright tones because of their purple-bluish component.

Herein is posted a figure of vegetables and fruits shot with a digital camera, converted in traditional b&w range, and shot in collodion.

Red peppers, yellow peppers, tomatoes and aubergines are evenly almost black. The red (bluish) grapes are rendered brighter and become similar to the white (pale yellow). The green part of the vegetables vary in its content of yellow and green ad is for that reason more or less brilliant. The purple component in the red part of the leafs of the "radicchio rosso" (salad leaf) are brighter that expected. The yellow pepper on the left, displays in collodion its shrinker surface, lost for poor contrast in the color image. White remain white. Black or red surfaces react almost only for reflected light. So the peppers in relative shadow, don't have texture in there darkest part. If I have increased the exposure I could have had detailes in that regions, but with heavy overexposure of the shells, one of them is already confounding with the surface of the table.

This remarks are true also for chemically similar processes such as calotypes or daguerreotypes which generate a negative or a one-of-a-kind positive image (that actually remains a negative, even if can be appear different).

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