Light matters. It’s important to understand the influence of light: how to control it, how to compensate when you can’t, and what the limits are.
The Fine Art of Printing
Light does matter. It’s important to understand the influence of light: how to control it, how to compensate when you can’t, and what the limits are.
- Implement colour management as precisely as practical; for example, control the printing and display environment. Control the light by using a full smooth spectrum light source with a desired colour temperature. Change the light, or when you can’t, recommend an appropriate light source for display.
- When there’s an alternate viewing environment or a profile mismatch, you can compensate for a mismatch in light temperatures. Evaluate and adjust prints to look correct under a light temperature similar to the one under which they’ll ultimately be viewed.
Amount of light
First, let’s look at amounts of light, which are typically specified in units of lux (candelas per square meter) or footcandlelas; there are approximately 11 units of lux per footcandela. Alternately, lumens measure quantity as perceived by the human eye. Lighting between outdoor, indoor and museum levels vary roughly by a factor of 10-daylight is 100x indoor lighting and 1000x museum lighting.
The more light you have, the better your prints will look; but too much light may produce glare and eyestrain. Ironically, while you need ample light to see prints now, reduce their exposure to light and for long-term storage, store prints in the dark.
While light has many important qualities, two are particularly significant: temperature and special distribution. Differences in white light are commonly described by colour temperature. A light source’s colour temperature is determined by comparing its hue with the hue of a theoretical blackbody radiator. The more you heat the object, the more it glows and its colour changes.
Temperature is rated in Kelvin degrees-the lower the number, the warmer the light; the higher the number, the cooler the light. Manmade light sources vary in temperature dramatically and change over time, for example: tungsten 2800k, halogen 3600K, fluorescent 5000K, etc. The changing appearance of the sun as it passes across the sky can be rated on the same Kelvin scale: sunrise/sunset 200K, morning/afternoon 4300K, noon 5400K, overcast 6000K and shade 8000K. This is further modified by environmental factors, such as moisture or pollutants, when the colour temperature of most lighting scenarios changes.
An interesting relationship exists between brightness and colour temperature: as colour temperature increases, less light is required to generate a comparable brightness. Add to this, changing physiology. For instance, when the eye’s iris closes, light levels lower, shifting response away from cones toward rods, which are more sensitive to blue frequencies, thus causing whites to appear bluer. Colour adaptation-the rebalancing of the eye to changing light conditions, typically with reference to white is a psychological change. Even with colour constancy in play, the colour temperature of the viewing light will significantly influence the appearance of colours, particularly neutrals, including the very important white substrate base.
The industry standard for viewing light is 5000K, simulating daylight on a clear midday-a tradition that was adopted from offset reproduction. It’s useful because adopting it makes colour communication more precise. As most printer profiles are optimized for the 5000K standard, it’s particularly good light source for evaluating the quality of those profiles.
Most prints are viewed under light temperatures warmer than 5000K, typically a mix of tungsten (2800K) and daylight (variable). Galleries and museums favour halogen (3600K), as studies suggest that more people prefer viewing artwork under warmer light temperatures (3600K).
It’s a good idea to have two light sources of different colour temperatures (5000K and 3600K) for comparison. Evaluate proofs and make prints for the light temperature at which they’ll ultimately be viewed. If a light source is too cool or warm, a print will appear too blue/cyan or yellow/red. While you can make printing profiles for other viewing light temperatures, the vast majority of printing profiles are optimized for 5000K viewing light. (A few manufactures, such as Colourbyte, provide these profiles.) If you choose to make prints for temperatures other than 5000K with printing profiles optimized for 5000K, you’ll want to adjust the hue of your images before printing.
When using 5000K printing profiles, a common adjustment for moving to a 3600K viewing light is to add blue and cyan. Make prints that are too cool for a viewing light that’s too warm.
The spectral distribution of light isn’t often discussed. You can have a lot of light with an appropriate colour temperature and still not achieve true colour accuracy. To see colour accurately, a light source needs to be full and smooth spectrum, or contain all the colours of the rainbow in equal amounts.
The only natural full smooth spectrum light source is the sun: daylight is a combination of direct sunlight and sky light. Solux (ww.solux.net) manufacturers a full smooth spectrum manmade light source that most accurately reproduces daylight. Colour Rendering Index (CRI) ratings are used to describe the quality of light and Solux CRI ratings are 99 on a scale of 100.
Light sources that are not full smooth spectrum contain an uneven distribution of colours, shown in graphs by spikes in specific regions of the spectrum. The spikes limit the number of available colours in a spectrum to discern an object's colour. Missing colours in between spikes make objects colour appear dull or grey. When a spectrum is uneven, hues that are found in elevated levels appear brighter, while hues that are found in low levels appear duller. Spikes also create an imbalance in the relationships between hues.
Incandescent light contains large amounts of yellow, orange and red light, and warm colours appear brighter than cool colours. Though not as extreme, halogen suffers from the same tendencies. Cool white fluorescent light may produce a white that’s cooler in appearance but it has a different uneven spectral distribution.
Get good light: it makes your prints appear even more beautiful. It’s one of the most essential elements in any photographic image at the point of capture, during processing and at the point of display.