Color Temperature

Color temperature names a light by the temperature of an ideal glowing body that would have the same color. A hotter body shifts its output toward blue. For light that is not thermal, such as an LED, the figure is a correlated color temperature: the nearest match on that scale.

The scale describes real spectral differences, not just a tint. From Planck's law, a 3,200 K source, typical of a microscope halogen lamp, emits only about 0.29 times as much at 450 nm (blue) as at 650 nm (red). At 5,500 K the two are about equal, and at 6,500 K, a common daylight standard, blue is about 1.35 times red. A camera under halogen therefore collects roughly a quarter as much blue as red from blank glass.

An incandescent or halogen filament's temperature depends on the power it dissipates, so color temperature follows lamp voltage. Lamp engineering's rules of thumb put color temperature at about V^0.42 and light output at about V^3.4. For a lamp rated 12 V at 3,200 K: at 10.5 V it runs near 3,025 K with 64% of the light; at 9 V, near 2,840 K with 38%.

So dimming a halogen lamp to a third of its brightness also cools it by about 360 K, and a white balanceLoading... set at full voltage no longer fits. The image turns yellow, and stain measurements shift with it. That is why the voltage belongs in the acquisition record.

Keep the lamp at one voltage, the one you white-balanced at; some microscopes mark it. Control brightness with a neutral density filterLoading..., which removes light evenly across the spectrum, or with the exposure timeLoading..., which the camera applies identically to every channel.

LED illuminators hold their color far better as current changes, but they are not free of it. A white LED is a blue emitter plus a phosphor, so its spectrum has a strong blue peak and a dip in the blue-green, and its color drifts slowly with temperature and age. Different LED models also differ from each other. White-balance each illuminator, and re-check over months.