In my last article, I talked about some of the basic technical aspects that we need to think about when we start on the lighting trail. I covered light levels and intensities in relation to lens apertures before discussing colour temperature and its relevance to producing ‘nice’ pictures. Although I’d like to move on into lighting ‘proper’ there are some more ‘techie’ things that need to be understood if our lit pictures are to be of the highest quality.
Any surface (including skin tones) as seen by a camera will usually have a range of tones and shades of colour by virtue of its nature. This is known as the inherent contrast ratio, being the ratio between the luminance of the lightest and darkest part of the subject or image. A typical scenic contrast outdoors on an overcast day has an average of about 160:1, although on a sunny day it may be as high as 1000:1.
In years gone by, our dear old film cameras coped well with such high contrasts thanks to a forgiving transfer characteristic, especially in highlight areas. But as you will be well aware, once you have ‘blown’ your highlights in video, they are gone for ever. So if we’re using ‘chips’ and tape, we need to restrict the contrast range of the lit scene.
In a studio situation, the contrast ratio is usually under the control of the Production Designer who will be aware of the different tonal values of the paints and materials he uses and how they affect the television picture. Experience will also enable him to visualise how the set can be lit to good effect. I have always felt that the relationship between a lighting director/D.O.P. and a designer is a very important one as the end result is heavily dependent on working closely together.
An acceptable contrast ratio (ACR) defines the contrast range over which it is possible to accurately reproduce the tonal range of the original scene. The television system using electronic cameras and gamma correction can handle an ACR of about 32:1 between the extremes of peak white limiting and black crushing. This translates as being the ability to handle about 5 F stops; useful to remember when you’re on location.
The actual contrast ratio will be greater than the inherent contrast ratio when some areas of the scene are in a reduced light level, or the surfaces are at an angle to the light source. This aspect is under the control of the Lighting Director/D.O.P. who can, therefore, enhance (or spoil!) the work of the Production Designer. Of course, the actual contrast ratio of faces and clothes that an artist wears can also cause the ACR to be exceeded.
Imagine a black presenter in a white suit or a pale white person with a black velvet jacket and white nylon shirt! It happens! Both are outside our comfort zone of 32:1 and on camera you would expect to lose detailed information in both materials if the face tones are correctly exposed.
In television, because electronic cameras cannot (yet) handle the enormous contrast ratios that nature presents, we try and ensure that the inherent contrast ratio should be restricted to about 20:1. We do this by ensuring that the colours used by Scenic Designers are selected from a special range of paints and finishes, which have reflectance between 3 and 60%. The L.D./D.O.P. can then use creative lighting to increase that ratio to the maximum for TV at 32:1.
‘Television white’ is a surface that has a reflectance of 60%; the average European face is approximately half this value i.e. 30%. Note that gamma correction (which all cameras use) enhances that value to approximately half a volt of TV waveform, where 0.7V equates to peak white.
‘Brilliant white’ paint out of a tin bought at your local DIY store is a most definite no, no! But, I can hear you saying, ‘as a lighting cameraman I often have to walk into and light a room where the walls might well be off the scale of
‘acceptable TV values.’ Hang on in there, I promise to cover that and other difficult situations in the fullness of time. Location lighting where you have little or no control over what is seen by your camera is tricky, but there are ways and means of achieving good results.
Monitor Line up
After some twenty years lighting in the BBC, I tended to take camera and monitor line up for granted, but when working in the freelance sector, I realised that there are large variations in standards.
Before becoming a Lighting Director I worked in vision control, where one had to line up cameras and monitors very carefully every working day. In the early days of colour TV, much of the equipment was valve rather than solid state, so stability was always a problem. We would line up monitors in the key areas several times a day (using a spot meter and a test signal with the acronym PLUGE (Picture Line Up Generating Equipment) and be constantly checking camera line up with small ‘tweaks’ even during transmission or recording!
Thankfully, all electronic equipment has become much more stable, and it is usually sufficient to line up once before rehearsals. Having said that, it is important to go through this process because without a known standard of monitor line up, how can you know what your camera output is like? If you are a photographer and you print out your work at home, you’ll be familiar with the similar problem of making your prints look similar to the screen image.
The BBC has long maintained that all the key members of the programme realisation team should view the same monitor when making decisions about colour saturation or brightness in relation to their area of expertise, whether it is set design, make up or costume. They are the L.D./D.O.P., the Production Designer, and the Make-up and Costume Designers. Although they have access to other monitors in their areas of work, they should (and do) always come to see a grade one monitor in the lighting gallery to make their final assessment of those qualities.
Camera Line up
Studio cameras should be ’standardised’ by aperiod ofline-up, so that they give identical matching pictures on the same test chart. The chart or white card should be evenly illuminated with the line up light, which should normally be set up at 70% of its maximum light output by the dimmer, and the flood/spot mechanism used to give the required light intensity (chosen by the L.D.) at the line up chart. Why 70%, I am often asked?
Studio lighting control faders are usually calibrated between 0 and 10. Most modern electronic dimmers also have a choice of various laws, which can be selected by a switch. If a ‘square law’ is selected then 50% of light output (in Lux or Foot Candles) corresponds with a fader setting of ‘7’ (or 70%). As far as I know, the BBC still operates with a square law in all studios.
Moving the fader smoothly from 0 to 10 will produce an apparently linear increase of light from minimum to maximum. This satisfies the logarithmic nature of the eye/brain combination in that it will perceive linear increases of light for linear increments of the fader (see Fig1).
Using the lamps set in this way ensures an acceptable colour temperature for the representation of ‘white’ of about 2850K, which, in turn, enables all lamps to have 70% as their ‘normal’ setting. This has important operational and financial advantages.
Lamp life is greatly extended from about 150 hours (at full mains voltage) to somewhere in the region of 1500 hours. A significant saving in lamp replacement costs results.
It will now be possible to increase or decrease the amount of light produced by any lantern (the correct name for the whole fixture) simply by adjusting the fader up to 100 or down to 50% on the fader. A variation of +/- 150Kelvin will result, which in most circumstances is usually quite acceptable. (Having said that, in a portraiture situation one should be wary of using two lamps at the extremities of colour temperature described on the same person) The practical benefit of this is that the L.D.can increase or decrease the brightness of the lamp (within the limits above) without leaving his seat. This aids the process of creating even intensities when the subject is lit by a number of lamps [e.g. an audience or a long walk by an artist].
Some years ago, I was asked to visit a large studio somewhere in Europe to see if I could make suggestions for reducing running costs. They were replacing several lamps (bulbs or even more colloquially: ‘bubbles’) a week at a cost of many Euros. A simple calculation showed that if they adopted the square law response on their dimmers and used 70% as a new ‘norm’ for dimmer settings, they could afford to buy a new camera channel with lens in one year with the money saved! OK, so the camera lenses would need to be at F2.8-4 instead of F5.6-F8, but that wasn’t too serious an issue. In fact, the director would probably appreciate a little more selective focus on some of his shots.
It is worth noting that another way of increasing the flexibility of studio lighting is to use lanterns that utilise dual filament lamps. Typically, a lamp can have a 1.2Kwatt and a 2.5Kwatt filament in the same envelope. It is then possible to switch between three powers: 1.2K, 2.5K, and 3.75K, just by switching through the two filaments separately or together. In today’s cost-conscious world this can reduce the need for movement of lanterns.
Choosing an operating light intensity
In a BBC studio, a mean figure might typically be 600 lux at a colour temperature of 2850K. A vision operator would set a dedicated ‘Line-up light’ onto a gray scale in such a position that all cameras could provide a full frame image. The lamp would be set at 70% using its dimmer control, and the flood/spot mechanism would be adjusted to get a reading of 600 lux at the centre of the chart (having checked that the light is as far as possible tangential to the chart).
When the vision engineers are happy with the technical line up, the cameras are ‘handed over’ to the vision staff in the studio gallery who will check the cameras for any slight matching problems that can be corrected by small adjustments to gain, lift gamma and sharpness on the operational control panels.
Once rehearsals begin, the cameras are operated at the iris setting established during line-up (typically F4). It is then the task of the Lighting Director to adjust his lighting ‘balance’ to produce a correctly exposed picture. The Vision Operator should not normally make corrections to the camera exposure or colour balance at this stage because line up has established a standard exposure. The V.O. can look but not touch!
It is a fundamental difference between photography and TV that the camera exposure is adjusted to suit the lighting conditions in photography, whereas in TV studio lighting the camera exposure is kept constant and we adjust the light levels. This is clearly not so easy on location shoots!
As I have progressed from my studio lighting background onto location lighting over the last 15 years or so, you can be sure that I will cover location lighting in some detail in future articles. If you’re a bit impatient, you might like to consider attending a five day location lighting training course that an ex-BBC colleague, Bernie Davis, and I run in Gloucestershire. Check out http://www.bakerlite.co.uk/training.htm for more details.
For the time being, I feel the knowledge of working practices in a ‘black box’ or studio situation are not out of place for anyone who is really interested in the craft of lighting.
Having covered some of the techie side of lighting, I intend to move on to the real world. Theory isn’t very exciting and I am one of the least techie lighting people around, but it does set the scene for future articles. I would like to give acknowledgment for some of the facts and figures as well as the graph and chart to Alan Bermingham, another ex-BBC colleague who has a much better grasp of theory and techie things than myself!
My next article will cover some of the ‘rules’ of portrait lighting. Like all rules, you don’t always have to ‘obey’ them, but you do need to know them in order to break them!