How color negatives work
A developed color negative is not a picture — it is a measurement. Each point on the film stores how much light hit it, recorded as density on three dye layers, behind an orange mask.
Three dye layers
Color film has stacked emulsion layers sensitive to blue, green, and red light. After development each layer carries a complementary dye — yellow, magenta, and cyan. More scene light means more dye, means more density. That inversion (bright scene → dark negative) is why negatives look "backwards."
Why the base is orange
The magenta and cyan dyes are imperfect — they absorb some light they shouldn't. To compensate, color-negative film adds an orange mask: a built-in colored base that corrects the dyes for accurate printing. That is why the clear, unexposed edge of the film is orange, not transparent.
OpenEnlarge samples the base color from the film and divides it out — this is the first step of inversion. See film-base calibration.
The H&D characteristic curve
The relationship between scene exposure and film density is the Hurter–Driffield (H&D) curve. It has a gentle toe in the shadows, a straight mid-section, and a shoulder where highlights compress. This shape — not a straight line — is the "film look," and OpenEnlarge's tone curve is designed to invert it faithfully. See the tone curve.
Under the hood
Density is logarithmic: D = log₁₀(base / scan) Here base is the pixel value sampled from the clear film edge and scan is the raw value at each point — dividing cancels the mask and leaves density. One density unit corresponds to about 3.32 photographic stops of scene brightness. Typical orange-mask transmission ratios are roughly R : G : B ≈ 1 : 0.55 : 0.26, which is why the clear base reads as orange.