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.

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Colour Rendering Index

Colour Schemes

Last week I looked at the science of colour: what it is, how our eyes see it, and how cameras see and process it. Now I’m going to look at colour theory – that is, schemes of mixing colours to produce aesthetically pleasing results.

 

The Colour wheel

The first colour wheel was drawn by Sir Isaac Newton in 1704, and it’s a precursor of the CIE diagram we met last week. It’s a method of arranging hues so that useful relationships between them – like primaries and secondaries, and the schemes we’ll cover below – can be understood. As we know from last week, colour is in reality a linear spectrum which we humans perceive by deducing it from the amounts of light triggering our red, green and blue cones, but certain quirks of our visual system make a wheel in many ways a more useful arrangement of the colours than a linear spectrum.

One of these quirks is that our long (red) cones, although having peak sensitivity to red light, have a smaller peak in sensitivity at the opposite (violet) end of the spectrum. This may be what causes our perception of colour to “wrap around”.

Another quirk is in the way that colour information is encoded in the retina before being piped along the optic nerve to the brain. Rather than producing red, green and blue signals, the retina compares the levels of red to green, and of blue to yellow (the sum of red and green cones), and sends these colour opponency channels along with a luminance channel to the brain.

You can test these opposites yourself by staring at a solid block of one of the colours for around 30 seconds and then looking at something white. The white will initially take on the opposing colour, so if you stared at red then you will see green.

Hering’s colour wheels

19th century physiologist Ewald Hering was the first to theorise about this colour opponency, and he designed his own colour wheel to match it, having red/green on the vertical axis and blue/yellow on the horizontal.

RGB colour wheel

Today we are more familiar with the RGB colour wheel, which spaces red, green and blue equally around the circle. But both wheels – the first dealing with colour perception in the eye-brain system, and the second dealing with colour representation on an RGB screen – are relevant to cinematography.

On both wheels, colours directly opposite each other are considered to cancel each other out. (In RGB they make white when combined.) These pairs are known as complementary colours.

 

Complementary

A complementary scheme provides maximum colour contrast, each of the two hues making the other more vibrant. Take “The Snail” by modernist French artist Henri Matisse, which you can currently see at the Tate Modern; Matisse placed complementary colours next to each other to make them all pop.

“The Snail” by Henri Matisse (1953)

In cinematography, a single pair of complementary colours is often used, for example the yellows and blues of Aliens‘ power loader scene:

“Aliens” DP: Adrian Biddle, BSC

Or this scene from Life on Mars which I covered on my YouTube show Lighting I Like:

I frequently use a blue/orange colour scheme, because it’s the natural result of mixing tungsten with cool daylight or “moonlight”.

“The First Musketeer”, DP: Neil Oseman

And then of course there’s the orange-and-teal grading so common in Hollywood:

“Hot Tub Time Machine” DP: Jack N. Green, ASC

Amélie uses a less common complementary pairing of red and green:

“Amélie” DP: Bruno Belbonnel, AFC, ASC

 

Analogous

An analogous colour scheme uses hues adjacent to each other on the wheel. It lacks the punch and vibrancy of a complementary scheme, instead having a harmonious, unifying effect. In the examples below it seems to enhance the single-mindedness of the characters. Sometimes filmmakers push analogous colours to the extreme of using literally just one hue, at which point it is technically monochrome.

“The Matrix” DP: Bill Pope, ASC
“Terminator 2: Judgment Day” DP: Adam Greenberg, ASC
“The Double” DP: Erik Alexander Wilson
“Total Recall” (1990) DP: Jost Vacano, ASC, BVK

 

There are other colour schemes, such as triadic, but complementary and analogous colours are by far the most common in cinematography. In a future post I’ll look at the psychological effects of individual colours and how they can be used to enhance the themes and emotions of a film.

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Colour Schemes

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

Period Cinematography

White "daylight" (a 2.5K HMI outside the window and  a Kinolfo Barfly behind the actor) and warm "candlelight" (a Dedolight off camera right)
White “daylight” (a 2.5K HMI outside the window and a Kinolfo Barfly behind the actor) and warm “candlelight” (a Dedolight off camera right)

The First Musketeer was my first period production as DP. It’s a genre that brings its own set of challenges and opportunities, most obviously for sets and costumes, and also sound (we spent a lot of time waiting for cars and planes to pass by), but for cinematography too. The first thing that hit me was the restrictiveness of it. Back in the day there were only three sources of light: the sun, the moon and fire. And maybe, at a pinch, starlight.

 

Blue "moonlight" and orange "firelight" - in this case both created by gelled Dedolights
Paul McMaster as Ghislain. Blue “moonlight” and orange “firelight” – in this case both created by gelled Dedolights

I kept colour temperatures simple by deciding that daylight would always appear white, moonlight would be +2,400K (blue) and firelight would be -2,400K (orange). In practice this meant that daylight scenes were white-balanced at 5,600K using natural light, HMIs and kinoflos, with ungelled redheads or dedos for candlelight, while night scenes were typically white-balanced at 3,200K which turned HMIs and kinos blue for moonlight/starlight, with redheads or dedos gelled with full CTO to turn them orange on camera.

This night exterior shot of Lazare (Tony Sams) and Athos (Edward Mitchell) was shot with a white balance of 3,200K, turning the HMI backlight blue, while the warm light around the taven entrance was provided by CTO-gelled Dedos and redheads.
This night exterior shot of Lazare (Tony Sams) and Athos (Edward Mitchell) was shot with a white balance of 3,200K, turning the HMI backlight blue, while the warm light around the tavern entrance was provided by CTO-gelled dedos and redheads.

Occasionally I used straw gels to give “firelight” more of a yellow hue than an orange one, and in one scene involving a church I introduced strongly yellow light and some pink backlight, the theory being that stained glass windows could be held accountable.

A 2.5K provides the frontal keylight here, while a redhead sporting Minus Green gel provides the pink backlight. A second redhead double-gelled with Light Straw uplights the figure of Christ on the back wall, and finally a 1.2K HMI at the rear of the building illuminates the stained glass window.
A 2.5K provides the frontal keylight here, while a redhead sporting Minus Green gel provides the pink backlight. A second redhead double-gelled with Light Straw uplights the figure of Christ on the back wall, and finally a 1.2K HMI at the rear of the building illuminates the stained glass window.

I think it’s very important to soften the images when shooting a period piece digitally. Initially we hoped to do this by using Cooke lenses, but they proved unobtainable on our budget. It was too late to look into filters by this point, so instead I relied on smoke in most scenes to diffuse and age the image.

Like everyone, I continue to learn with every project that I do. Reviewing the rushes towards the end of the shoot, I realised (a little too late) that texture was the key to making the period convincing. There was bags of it in front of me – in the stone walls of the locations, in the beautifully-aged costumes, in the detailed set dressing. It was an era before smooth surfaces. I can now see that my cinematography was most successful when the lighting brought the textures out.

A 1.2K HMI outside the door cross-lights the stonework, while smoke volumizes this light, resulting in a very satisfying depth and texture. The only other light sources are two kinoflo Barflies hanging from polecats above the bench at the back of shot. This backlight is reflected back at the foreground characters by a sheet of silver foamcore beneath the camera.
A 1.2K HMI outside the door cross-lights the stonework, while smoke volumizes this light, resulting in a very satisfying depth and texture. The only other light sources are two Kinoflo Barflies hanging from polecats above the bench at the back of shot. This backlight is reflected back at the foreground characters by a sheet of silver foamcore beneath the camera.

Contrast the shot above with the one below. This location had equally nice stonework, but because I didn’t cross-light it it looks flat and artificial, like a cheap panto set.

A 2.5K HMI supplies the backlight here, while a blue-gelled redhead out of the top right of frame is aimed down the steps to pick out the characters as they descend. An orange-gelled Dedo creates a pool of light around the candle, and everything else is natural bounce off the surrounding stonework. A second blue-gelled redhead at the foot of the stairs firing across the stonework would have made all the difference to the believability of the environment, but hindsight is 20/20.
A 2.5K HMI supplies the backlight here, while a blue-gelled redhead out of the top right of frame is aimed down the steps to pick out the characters as they descend. An orange-gelled Dedo creates a pool of light around the candle, and everything else is natural bounce off the surrounding walls. A second blue-gelled redhead at the foot of the stairs firing across the stonework would have made all the difference to the believability of the environment, but hindsight is 20/20.

So that’s an important lesson I’ve learnt to take forward to the next season. Next time around I also want to play more with different colours of daylight, using more straw, amber and pink gels to stretch out the colour palette and suggest different times of day.

And then there’s the whole candlelight thing – but I’ll save that for my next post.

All images copyright 2013 The First Musketeer. Find out more about the series at www.firstmusketeer.com

Period Cinematography