Tungsten Correction

Overview

"Tungsten Correction" (also Daylight-to-Tungsten Correction) refers to adjusting a daylight-balanced light source to match the color temperature of artificial light (Tungsten/incandescent, standard 3200 K). A classic scenario: an HMI or LED source balanced for daylight (approx. 5600 K) is to be used indoors with existing incandescent lamps or together with tungsten fixtures, without creating a color break in the image between bluish and warm light.

The usual tool is a CTO gel (Color Temperature Orange), which is stretched in front of the source. It absorbs parts of the blue spectrum and shifts the color temperature downwards, towards warmer tones. The counterpart is CTB (Color Temperature Blue), which conversely raises tungsten to daylight. With LEDs and many HMIs, this correction can today also be achieved electronically via the preset color temperature; however, the gel remains the reference method, especially for non-dimmable sources and mixed lighting situations.

Technical Data

CTO gels are available in graduated densities. The following conversions correspond to the manufacturer specifications from LEE Filters (starting value 6500 K daylight):

The effect is often specified in Mired Shift (positive value = warmer). Mired values can be added, allowing the necessary correction for a specific source and target color temperature to be combined. With Rosco, CTO gels are sometimes referenced to a different starting color temperature (approx. 5500 K), which is why target values may differ slightly between manufacturers. Related are the "Straw" variants (LEE 441-444), which additionally introduce a slight yellow cast.

On-Set Usage

• Matching Mixed Light: Daylight HMI coming through a window or used as fill is brought to tungsten level with Full CTO so it matches practical incandescent lamps or tungsten key lights.
• Partial Correction: Fractions (1/4, 1/2) only compensate for part of the difference or selectively warm the light, for example, to create an afternoon or evening mood.
• White Balance: If consistently corrected to tungsten, the camera can be set uniformly to 3200 K.

Light loss must be considered: the denser the CTO gel, the more output it absorbs. With electronically dimmable LED sources, this loss is avoided because the correction is applied directly via the light color.