Colour Correction Filters – More Stage Lighting Gels

Colour correction lighting filters are an easy solution to achieve a standardised white light. Following on from Colour Temperature …, this article looks at colour correction filters and their uses in lighting design, both practical and artistic.

What is colour correction?

Colour correction is the altering of a light sources colour temperature, to achieve a certain light quality or to provide cameras with an regular colour balanced white light. Before we go any further, make sure that you learn about colour temperature…

What do colour correction filters do?

All stage lighting colour gels block certain parts of the colour spectrum of light. Colour correction filters block out the specific wavelengths to change the colour temperature of one light source, to another one. A common use in stage lighting is to “correct” a stage light (tungsten light source) to daylight and there are a number of correction gels that will do this.

Colour filters stop light from passing through them, so the resultant light is reduced. Colour correction gels only reduce small amounts of colour so are fairly “pale” filters that allow as much light as possible through – with the desired colour temperature.

Comon Colour Correction Gels

Tungsten Light Source Correction – Colour Temperature Blue – The CTB range of blue correction gels convert Tungsten Light sources of 3200K to a range of colour, comparable to daylight. These can be used in stage lighting to make a key light appear to be from an outdoor source such as the sky or a window.

Lee 201 Full CTB : Tungsten to Daylight 3200K – 5700K
Lee 202 Half CTB : Tungsten to Daylight 3200K – 4300 K
Lee 203 Quarter CTB : Tungsten to Daylight 3200K – 3600K

Lee 219 Fluorescent Green – Correction to convert tungsten light to general fluorescent lighting.
Lee 242 Fluorescent 4300K – Correction to convert tungsten light to fluorescent @ 4300K.

Daylight Light Source Correction – Colour Temperature Orange – The CTO range of orange correction gels convert the blue-ish light source of daylight to a range of tungsten colour temperatures. These are useful for balancing light sources from a window with tungsten key lighting. CTO can also be used with a tungsten light to create a warm sunny keylight.

Lee 204 Full CTO – Daylight 6500 K to Tungsten 3200 K
Lee 205 Half CTO – Daylight 6500 K to Tungsten 3800 K
Lee 206 Quarter CTO – Daylight 6500 K to Tungsten 4600 K

Other Correction Gels

There are many colour correction filters available to balance different light sources such as HMI , CID and CSI arc lamps, which are all used in theatre, with other stage lighting. Again, these gels block different amounts of the light spectrum to correct for each light source but can be used as colour filters in “standard” stage lights. Here are some common correction filters and their “unusual” uses:

Lee 236 HMI to Tungsten 3200K – A nice warm orange tint that can be used for sunlight in tungsten lights
Lee 237 CID to Tungsten 3200K – Light red/brown. Used with tungsten, this gel produces a “olde” country summer feel.
Lee 238 CSI to Tungsten 3200K – Another great gel for creating a “rustic” look for Thomas Hardy plays.

Intelligent Lighting and Correction Filters

Moving lights use different arc lamps to produce a bright white light which doesn’t match up with the tungsten of other stage lighting. These colour temperatures can be correctec using correction filters built into the fixture by the manufacturer. Both Daylight and Tungsten colour temperatures are available from the fixture and these are remotely controlled from the lighting console.

Neutral Density Filters – ND

Neutral Density Filters are worth mentioning but they are, however, not colour correction gels. In fact, they couldn’t be more different.

While correction filters adjust the colour output of a stage light, Neutral Density leave the colour temperature intact but cut down the total light output – brightness. Nd filters are neutral grey and block the lighting spectrum evenly, creating a less bright version of the original source.

Reducing brightness – Isn’t that what dimmers do?

Using a dimmer to reduce the intensity of a stage light changes the colour temperature. Quite a lot. The warm glow of a lamp filament at 20% is a good deal more orange than our 3200K tungsten lamp at full brightness. This might not matter in your stage show but, as the Colour Temperature article expalins, lighting colour is very important to cameras.

Neutral density filters are used in the Photographic, TV and Film industry to adjust the brightness of a light source , because dimmers changes the colour temperature. This relationship with cameras explains why the intensity level that an ND filter control is measured in F stops – the standard expression photographers use to describe the aperture setting on a camera.

Some common ND filters are:

Lee 298 .15 ND – Reduces instensity 0.5 stop
Lee 209 .3 ND – Reduces intensity 1 stop
Lee 210 .6 ND – Reduces intensity 2 stops
Lee 211 .9ND – Reduces intensity 3 stops

Colour correction filters are practical gels that can be used to tightly control source colour temperatures but they can be chosen to just change the colour of your stage lights. Experiment with lighting and colour and to find the filters that you like. There are no rules in lighting design but understanding the basics of lighting colour theory is important to any stage lighting designer and colour correction is one of those basics.