Colour Rendering Index

Many light sources we come across today have a CRI rating. Most of us realise that the higher the number, the better the quality of light, but is it really that simple? What exactly is Colour Rendering Index, how is it measured and can we trust it as cinematographers? Let’s find out.

 

What is C.R.I.?

CRI was created in 1965 by the CIE – Commission Internationale de l’Eclairage – the same body responsible for the colour-space diagram we met in my post about How Colour Works. The CIE wanted to define a standard method of measuring and rating the colour-rendering properties of light sources, particularly those which don’t emit a full spectrum of light, like fluorescent tubes which were becoming popular in the sixties. The aim was to meet the needs of architects deciding what kind of lighting to install in factories, supermarkets and the like, with little or no thought given to cinematography.

As we saw in How Colour Works, colour is caused by the absorption of certain wavelengths of light by a surface, and the reflection of others. For this to work properly, the light shining on the surface in the first place needs to consist of all the visible wavelengths. The graphs below shows that daylight indeed consists of a full spectrum, as does incandescent lighting (e.g. tungsten), although its skew to the red end means that white-balancing is necessary to restore the correct proportions of colours to a photographed image. (See my article on Understanding Colour Temperature.)

Fluorescent and LED sources, however, have huge peaks and troughs in their spectral output, with some wavelengths missing completely. If the wavelengths aren’t there to begin with, they can’t reflect off the subject, so the colour of the subject will look wrong.

Analysing the spectrum of a light source to produce graphs like this required expensive equipment, so the CIE devised a simpler method of determining CRI, based on how the source reflected off a set of eight colour patches. These patches were murky pastel shades taken from the Munsell colour wheel (see my Colour Schemes post for more on colour wheels). In 2004, six more-saturated patches were added.

The maths which is used to arrive at a CRI value goes right over my head, but the testing process boils down to this:

  1. Illuminate a patch with daylight (if the source being tested has a correlated colour temperature of 5,000K or above) or incandescent light (if below 5,000K).
  2. Compare the colour of the patch to a colour-space CIE diagram and note the coordinates of the corresponding colour on the diagram.
  3. Now illuminate the patch with the source being tested.
  4. Compare the new colour of the patch to the CIE diagram and note the coordinates of the corresponding colour.
  5. Calculate the distance between the two coordinates, i.e. the difference in colour under the two light sources.
  6. Repeat with the remaining patches and calculate the average difference.

Here are a few CRI ratings gleaned from around the web:

Source CRI
Sodium streetlight -44
Standard fluorescent 50-75
Standard LED 83
LitePanels 1×1 LED 90
Arri HMI 90+
Kino Flo 95
Tungsten 100 (maximum)

 

Problems with C.R.I.

There have been many criticisms of the CRI system. One is that the use of mean averaging results in a lamp with mediocre performance across all the patches scoring the same CRI as a lamp that does terrible rendering of one colour but good rendering of all the others.

Demonstrating the non-continuous spectrum of a fluorescent lamp, versus the continuous spectrum of incandescent, using a prism.

Further criticisms relate to the colour patches themselves. The eight standard patches are low in saturation, making them easier to render accurately than bright colours. An unscrupulous manufacturer could design their lamp to render the test colours well without worrying about the rest of the spectrum.

In practice this all means that CRI ratings sometimes don’t correspond to the evidence of your own eyes. For example, I’d wager that an HMI with a quoted CRI in the low nineties is going to render more natural skin-tones than an LED panel with the same rating.

I prefer to assess the quality of a light source by eye rather than relying on any quoted CRI value. Holding my hand up in front of an LED fixture, I can quickly tell whether the skin tones looks right or not. Unfortunately even this system is flawed.

The fundamental issue is the trichromatic nature of our eyes and of cameras: both work out what colour things are based on sensory input of only red, green and blue. As an analogy, imagine a wall with a number of cracks in it. Imagine that you can only inspect it through an opaque barrier with three slits in it. Through those three slits, the wall may look completely unblemished. The cracks are there, but since they’re not aligned with the slits, you’re not aware of them. And the “slits” of the human eye are not in the same place as the slits of a camera’s sensor, i.e. the respective sensitivities of our long, medium and short cones do not quite match the red, green and blue dyes in the Bayer filters of cameras. Under continuous-spectrum lighting (“smooth wall”) this doesn’t matter, but with non-continuous-spectrum sources (“cracked wall”) it can lead to something looking right to the eye but not on camera, or vice-versa.

 

Conclusion

Given its age and its intended use, it’s not surprising that CRI is a pretty poor indicator of light quality for a modern DP or gaffer. Various alternative systems exist, including GAI (Gamut Area Index) and TLCI (Television Lighting Consistency Index), the latter similar to CRI but introducing a camera into the process rather than relying solely on human observation. The Academy of Motion Picture Arts and Sciences recently invented a system, Spectral Similarity Index (SSI), which involves measuring the source itself with a spectrometer, rather than reflected light. At the time of writing, however, we are still stuck with CRI as the dominant quantitative measure.

So what is the solution? Test, test, test. Take your chosen camera and lens system and shoot some footage with the fixtures in question. For the moment at least, that is the only way to really know what kind of light you’re getting.

SaveSave

SaveSaveSaveSave

SaveSave

SaveSave

SaveSave

SaveSave

SaveSave

SaveSave

SaveSave

SaveSave

Colour Rendering Index

Know Your Lights: Tungsten

Following on from my ‘Know Your Lights’ overview last week, today I’ll look in more detail at the first category of lamps and the various units available and when you might use them.

And that first category is incandescent lighting, commonly known as tungsten. It is the oldest, simplest and most robust lighting technology. Tungsten lamps are the cheapest to hire, the easiest to repair, and emit a smoother spectrum of light than any other artificial sources, making for the most natural skin tones. For my money, there’s no better way to artificially light a human face than by bouncing a tungsten source off polyboard.

This comparison of domestic bulbs against daylight shows the spectra of light they emit.
This comparison of domestic bulbs against daylight shows the spectra of light they emit, with incandescent (tungsten) and halogen producing the smoothest spectra of the artificial sources.

Tungsten lighting units can be sub-categorised by the style of reflectors and/or lenses in the heads…

 

Open-face

Lilliput 300W open-face lamp
Ianiro Lilliput, a 300W open-face light

Redheads draw 800W each
Generic 800W open-face light

Unknown
Arrilite 1000, a 1K open-face light

A blonde - a basic 2,000W tungsten lamp
Generic 2K open-face light

The simplest instruments are known as ‘open-face’ because they have no lens to focus the light. By far the most common units are the 800 Watt and 2,000 Watt models. These are often referred to as ‘redheads’ and ‘blondes’ respectively, though I strongly discourage these terms for reasons touched on here. 300W models – dubbed ‘Lilliputs’ by manufacturer Ianiro – are also available, as well as 1Ks and much larger models like the Mole-Richardson Skypan 5K and Skylite 10K.

While I have lit entire no-budget features with just open-face lights, on larger productions the uneven and unfocused nature of their light makes them a poor relation of other units on the truck. They are most likely to get fired into a bounce board or used to create a little pool of light somewhere in the deep background where finesse is not needed.

 

Fresnel

Arri 300W fresnel
Arri 300W fresnel

Filmgear 650W fresnel
Filmgear 650W fresnel

Mole Richardson 1K 'baby' fresnel
Mole Richardson 1K ‘baby’ fresnel

Arri T12, a 12K fresnel
Arri T12, a 12K fresnel

The fresnel lens was invented in the early 19th century by French physicist and engineer Augustin-Jean Fresnel in order to increase the focus and throw of lighthouse lamps. Today in the film industry, fresnel lenses can be found on tungsten, HMI and even LED fixtures.

Fresnel lens
Fresnel lens

Tungsten fresnels come in the following wattages: 150W, 300W, 650W (a.k.a. ‘tweenie’), 1K, 2K, 5K, 10K, 12K, 20K, 24K.

1Ks and 2Ks are sometimes called ‘babies’ and ‘juniors’ respectively, but confusingly those terms can also refer to whether they are the smaller location models or larger studio versions of the same wattage.

Though the fresnel lens reduces the light output a little, the beam is much more focused and can therefore create a shaft of light through smoke, which open-face lamps cannot. Hence I sometimes use tungsten fresnels to simulate hard sunlight when shooting on a stage. But beware that shadows cast by a fresnel can sometimes show up the ridges in the lens.

I often fire fresnels into bounce boards, and because their light is more focused they require less flagging to control the spill than open-face units.

On Heretiks we used numerous 300W and 650W fresnels to beef up candlelight, often placing tough-spun diffuser over them, dimming them down to warm up the colour temperature, and flickering them too.

 

Par (parabolic aluminised reflector)

Par 16 (birdie)
Par 16 (birdie)

Par 38
Par 38

Filmgear 4-light Minibrute
Filmgear 4-light minibrute

Mole Richardson 9-light maxibrute
Mole Richardson 9-light maxibrute

Par lights use a parabolic (shaped like half a rugby ball) reflector and a lens to produce a soft-edged oval pool of light. They are extremely common in theatres, but are often used in film and TV as well.

Unlike fresnel and open-face units, par cans are referred to not by wattage but by the diameter of the bubble in eighths of an inch. So a Par 16 (a.k.a. ‘birdie’) has a 2″ bulb.

Par cans come in the following sizes: 16, 20, 36, 38, 46, 56, 64. They also come with various internal specs which affect the width of the beam.

Par cans are good for throwing shafts of light. On The Little Mermaid I used them to simulate car headlights, and as practicals (i.e. they were seen on camera) to uplight banners at the circus.

Maxibrutes (a.k.a. ‘Molepars’) are banks of multiple par 64 (1KW) lights. They come in banks of 4, 6, 9, 12 or 24. They pop up in the background of music promos quite often, because they look cool and kind of retro. I used two 9-light Maxibrutes, bounced off the tent roof, to illuminate the big top in The Little Mermaid. Some DPs like to use Maxibrutes for backlight on night exteriors. If you’re using them direct, you’ll need at least a sheet of diff to prevent multiple shadows.

Minibrutes (a.k.a. ‘fays’) are similar, but use smaller par 36 (650W) lamps.

 

Other

Dedo, de-e-edo. Dedo come and me want go home.
Dedolite

Dedolites are compact units that use a unique lens system to produce very focussed, controllable light from (most commonly) 150W bulbs. They are widely available to hire, come with in-line dimmers, and are small and light enough to be rigged overhead or in tight spots. I often use them to beef up practicals.

ETC Source Four
ETC Source Four

Source Fours or (a.k.a. ‘lekos’) are ellipsoid reflector spotlights. They feature cutters which can be used to shape the beam, they can be hired with different lenses (some of which are zoomable), and they can be fitted with gobos to project patterns. They are good for stylised pools of light or for firing into distant bounce boards without spilling light elsewhere.

Spacelights
Spacelights

Spacelights are wagon-wheel configurations of three or six 1K lamps inside a cylinder of diffusion material. They are normally used in large numbers to provide ambient toplight on stage. Click here for a brief video introduction to spacelights.

1K Jem ball
1K Jem ball

Jem Balls, or China balls, resemble Chinese paper lanterns. They come in 22″ (up to 1KW) and 30″ (up to 2KW) sizes and produce a very soft light which I personally find is never bright enough.

Bare bulbs (usually referred to as ‘globes‘) in pendant fittings can be hung from overhead or hidden behind set dressing, perhaps to beef up practicals. On Ren: The Girl with the Mark and other projects I hid some globes behind furniture to enhance the pool of light from candles.

Finally, tungsten is usually the most desirable type of bulb to use in practicals. It is commonplace when shooting a daylight interior for a spark to go around replacing the energy-saver fluorescent bulbs in the table lamps with old-school tungsten ones. The colour is much nicer, the skin tones are better as noted above, and they can be dimmed to just the right level for camera.

 

I’m sure I’ve missed something out – please feel free to let me know on Facebook or Twitter! Next week: HMIs.

Know Your Lights: Tungsten

Know Your Lamps: Overview

Welcome to the first in a series of posts looking at the many types of lighting instruments in use on film and TV sets today. This is not intended to be an exhaustive or comprehensive list, but it will give you a good idea of your options, particularly if you’re moving up from smaller productions – where lighting kit is mostly borrowed – to larger ones, where you’re required to submit a lighting list to a rental house.

Some of the key considerations when choosing a lamp are:

  • Colour temperature – how orange or blue the light appears – see this post for more info
  • CRI – Colour Rendering Index – how full a spectrum of light is emitted, and therefore how accurately colours are rendered
  • Light quality – how hard or soft the light is
  • Power consumption
  • Hire cost

Lamps can be divided into categories according to the means by which they produce light. Here is an overview of the main types.

 

Redheads draw 800W each

Incandescent (view detailed post)

Incandescent lamps work by passing electrical current through a wire filament which becomes so hot that it glows. In the film industry they are generally referred to as ‘tungsten‘ units after the metal which the filament is made from. Common tungsten lamps include Dedolites, 1K ‘babies’ and open-face 800W and 2KW units (which have misogynous nicknames I shall not repeat here).

Pros: cheap, dimmable, extremely high CRI

Cons: very inefficient, get very hot, colour temperature changes when dimmed

Colour temperature: 3,200K

Light quality: generally hard (although certain units like Space Lights are softer)

 

HMI fresnel

HMI (view detailed post)

The HMI (hydragyrum medium-arc iodide) is the most common form of high intensity discharge lamp used in the industry. It operates by creating an electrical arc between two electrodes which excites a gas. You may occasionally hear about an MSR (medium source rare-earth), which is slightly different technology, but as far as a cinematographer is concerned MSRs and HMIs are the same. They require a ballast to ignite the arc and regulate the current and voltage.

Pros: good CRI, good match for daylight, efficient

Cons: only dimmable down to 50%, expensive, heads and ballasts sometimes hum or ‘squeal’, older bulbs can vary in colour, flicker issues at certain shutter angles with magnetic ballasts

Colour temperature: 5,600K

Light quality: hard

 

Kino4x4Fluorescent (view detailed post)

Fluorescent lamps are found almost everywhere today, as strip lights in supermarkets and offices, and energy-saver bulbs in the home. Similar in principle to HMIs, electric current causes mercury vapour to emit UV light which is translated into the visible spectrum by the phosphor coating on the tube. Kino Flo pretty much has the monopoly on fluorescent lighting for the film industry. Like HMIs, fluorescents require a ballast.

Pros: reasonable CRI from Kino Flos (appalling CRI from domestic/commercial fixtures), very efficient, get warm but not hot

Cons: limited dimming, high fall-off of light

Colour temperature: 5,500K and 3,200K tubes available

Light quality: soft

 

LED copyLED (view detailed post)

Gradually replacing tungsten as the most common lamps found on no-budget shoots, LED (light emitting diode) units contain semi-conductors that emit light when their electrons reconfigure. The technology is advancing rapidly, but there is currently a wide range of LED lamps on the market, varying greatly in price and corresponding quality.

Pros: extremely efficient, barely get warm, can run off batteries, almost fully dimmable, some models have adjustable colour temperature

Cons: CRI ranges from almost acceptable in the expensive models to downright shocking in the cheaper ones

Colour temperature: varies

Light quality: varies

 

Though there are other types of lighting, like xenon, metal-halide and HEP (high efficiency plasma), the above four are the main ones you will encounter on film and TV sets today. Over the next few weeks I’ll look at each of those types in more detail, listing many of the specific units available in each category and their applications.

By the way, if your budget is too tight to hire film lamps of any kind, you may want to check out my post on lighting without movie lamps.

Know Your Lamps: Overview