Gel Correction Is Measured in Mireds, Not Kelvin

Colour temperature does not behave the way the Kelvin numbers suggest. The gap between 3200 K and 3400 K looks small and is visible; the gap between 6000 K and 6200 K looks the same size and is nearly invisible. Gels are specified in mireds for exactly that reason.

What a mired is

A mired is a million divided by the colour temperature in Kelvin. The conversion turns an uneven scale into an even one, so that a given mired shift produces roughly the same visible change wherever you start.

From the gel calculator:

• 3200 K → 312.5 mireds (tungsten)
• 5600 K → 178.57 mireds (daylight)
• 2900 K → 344.83 mireds
• 6500 K → 153.85 mireds

The shift is the difference. Correcting a tungsten fixture to daylight is 178.57 − 312.5 = −133.93 mireds, which the calculator reports as a full CTB.

Which direction is which

The naming trips people up because it describes what the gel does to the light, not what it does to the number.

CTB — colour temperature blue — raises the Kelvin and lowers the mired value. Used to make a warm tungsten source match daylight.

CTO — colour temperature orange — lowers the Kelvin and raises the mired value. Used to warm a daylight-balanced source to match tungsten.

So 5600 K to 3200 K is +133.93 mireds and a full CTO — the exact mirror of the case above, which is a useful check that you have the direction right.

Why this matters on a community stage

Mixed sources are the normal condition in an amateur venue: some tungsten lanterns, some LED units, a practical lamp on set, and daylight through a window in the rehearsal room. Left uncorrected, the difference reads as one area of the stage looking dingy and another looking cold, and it is usually diagnosed as a dimmer problem.

Two common jobs. Matching a practical — a table lamp in the set — to the rig, which is usually a small correction on the lantern rather than the practical. And matching a daylight-balanced LED wash to tungsten profiles, which is a full CTO and the difference between a coherent stage picture and two separate ones.

Partial gels and the cost in light

Full CTB and CTO have half, quarter and eighth versions, and partial correction is often the right answer — particularly when you want the sources to sit closer together rather than to match exactly.

The trade is output. Colour-correction gels absorb light, and full CTB in particular takes a substantial bite — which is why a rig corrected to daylight often needs rethinking rather than just gelling. On a small community rig with limited units, that loss can matter more than the colour match.

Where the arithmetic stops

The calculator gives you the shift and the nominal correction. What it cannot do is tell you what the fixture actually produces — a tungsten lantern at 60% on a dimmer is not running at 3200 K, because dimming a filament lowers its colour temperature substantially. Nor does it account for LED fixtures, whose spectra are not well described by a single colour temperature at all.

Which means the number is a starting point for a plot, and the real decision is made by looking at the stage. As with everything in a lighting rig: the arithmetic is for planning, and the rigging, patching and anything touching mains is a matter for a competent person and the venue's own rules.

Reading a gel swatch book

Manufacturers state each correction gel's mired shift on the swatch, which is what makes the calculator's output directly usable. A gel listed at −131 mireds is the full CTB you need for the tungsten-to-daylight case above; the quarter version will be around a third of that.

Shifts also add. Two quarter CTOs stack to roughly a half, which is useful when the correction you want is not a stocked value — though every layer costs more light and more heat in the gel frame.

Gel is consumable

Colour-correction gel in front of a hot tungsten source fades and eventually burns through, and a faded gel is no longer applying the shift you calculated. On a long run that drift is real and gradual enough to be missed.

Checking gel between performances, and keeping spares cut for the units that run hottest, is ordinary practice. And gel changes happen on cold lanterns or with proper gloves — the frames get hot enough to burn.