Reading: Eran Dinur, The Complete Guide to Photorealism for Visual Effects, Visualization and Games (Focal Press, 2022), Chapter 7, pages 82-86. This page is a guide to that chapter, not a substitute for it.
Natural nighttime light, briefly
There are only two natural sources at night: the moon and the stars. The moon is a reflector, and even full, its luminance is a tiny fraction of the sun's - by apparent magnitude the sun is roughly 400,000 times brighter. Starlight is much weaker still, and the sky is rarely clear enough of cloud, humidity and pollution for it to contribute meaningfully.
Put simply, natural night light is too dark to photograph usefully without a long exposure or high ISO. Long exposures make beautiful sky photographs; subjects on the ground come out flat, desaturated and uninspiring.
So almost every night scene in film uses light that is in the scene - street lamps, lit windows and doorways, headlights, a flashlight, a lit cigarette - or movie lights faking natural night, the familiar "Hollywood moonlight". Architectural visualization and games do the same. That is why this section is short: nighttime photorealism is mostly a subject of artificial lighting.
The Purkinje effect
Worth knowing because it settles an argument. Hollywood moonlight is distinctly blue. Is that photoreal or invented?
The moon reflects sunlight, which is white, so you would expect moonlight to be white. In fact light reflected from the moon is slightly warmer than pure sunlight, because the lunar surface reflects long wavelengths more strongly than short ones.
But our eyes shift colors toward blue in low light. That is the Purkinje effect. So moonlight does not appear warm to us - it reads neutral or slightly bluish. The blue moonlight convention is not wrong; cinematographers just exaggerate it for effect.
Artificial lighting
Man-made lighting is a combination of many lamp types and an endless assortment of shades, diffusers, reflectors, filters and lenses. A fixture - also called a luminaire - can produce almost any intensity, color, shape and focus, and those variations set the mood of an artificially lit space.
Dinur is clear that real-world lighting design, across architectural, interior, stage, and film and TV lighting, is far beyond the book's scope. What he extracts is the part digital artists need in order to emulate man-made light and integrate believably.
Color temperature
The color of emitted light is measured in Kelvin. Sodium vapour lamps produce a saturated orange around 1700K. A CRT monitor produces a bluish light around 7000K. Household lamps mostly run between 2400K for standard incandescent and 6000K for cool white fluorescent.
Three cautions that catch people out:
- The scale runs backwards from intuition. Low Kelvin values are the colors we call warm; high values are the ones we call cool.
- Kelvin does not mean physically hotter. Many lamp types do not produce light with a burning filament at all. Fluorescent lights produce much less heat than incandescent and still have a higher Kelvin temperature.
- Kelvin says nothing about saturation. Highly saturated light is rare outside special effects and stage lighting, because it gives poor color definition. Most bulbs, warm or cool, produce fairly desaturated light.
Light intensity: lumens and lux
There is no single universally accepted unit, which is part of the confusion.
Watts used to work as a rough proxy. It is a power consumption unit, not a light measure, but when nearly every bulb was incandescent, higher wattage did mean brighter. Once bulbs arrived that produced more light at far lower wattage, the shorthand broke.
Lumen measures the total amount of light emitted by a source - luminous flux. It is the straightforward indicator of how bright a bulb is.
Lux measures the intensity of light landing on a surface. One lux is one lumen per square meter. Architects, interior designers and stage and film lighting people use lux because it corresponds directly to the outcome they want in a given situation.
Dinur gives a set of reference values worth memorising, because they calibrate your sense of scale:
- Direct daylight - around 12,000 lux
- Overcast day - roughly 1000 lux
- Typical office or supermarket - around 500 lux
- Corridors and stairways - around 100 lux
- A parking area - as dim as 50 lux
Common lamp types
- Incandescent - a burning filament in a vacuum. The workhorse since the 19th century and still the most common household lamp despite being inefficient. Warm range, roughly 2000-3000K.
- Halogen - a tungsten filament in a small transparent envelope filled with a halogen gas, iodine or bromine. Similar character to incandescent but runs at much higher temperatures and produces much brighter light. Common at home and in professional photography, film and stage work.
- Fluorescent - no filament. Electricity excites mercury vapour in a tube, which makes the tube's phosphor coating glow. Substantially more efficient than incandescent or halogen and gradually replacing them. Originally large tubes producing a cold bluish light around 5000K, used in hospitals, supermarkets, factories and offices. Compact fluorescent lamps now cover a much wider range of color temperature and fit most fixtures.
- Neon - predates fluorescent by about 25 years. Electricity ionises a gas in a tube, which emits colored light; the color depends on the gas, with neon giving orange, hydrogen red, helium yellow. The output is weak and poorly suited to lighting a space, but neon has been used for decades in signs and decoration. Now ceding ground to LEDs.
- Sodium-vapour - extremely efficient, ugly orange light, poor color definition. Streets, roads, parking areas, industrial spaces.
- LED - around since the early 1970s but limited to small red indicator lights until blue and white LEDs and big increases in output arrived. Now the most versatile type: RGB units covering a wide range of color and intensity, running from flashlights through household lighting to giant advertising screens.
Light modifiers - the most useful idea here
Just as the atmosphere transforms sunlight, modifiers transform artificial light. Shades, diffusers, reflectors, filters and lenses.
Bare fluorescent tubes are normal in industrial spaces, but homes are rarely lit by naked bulbs - they are uncomfortable to look at and throw a harsh, unpleasant light. Shades have been used to diffuse and soften electric light from the beginning, and now exist in every shape and material: plastic, glass, linen, burlap, metal, paper. Opaque shades pair with reflectors to direct and intensify. Lenses create focused beams in some domestic fixtures and many stage lights. Soft boxes and diffusers in photography and cinematography create larger, softer emitters.
Then the sentence that matters most:
From the point of view of photorealism, modifiers are more important than the bulbs. A given lamp produces a particular intensity and color temperature, but the eventual shape, pattern, color, intensity and softness of the fixture are set by its modifiers. An incandescent bulb inside a square diffuser produces a square soft light even though the bulb is spherical.
So when emulating man-made lighting, look at the characteristics of the whole fixture and its final output, not at the lamp specification.
Terms from this chapter
- Purkinje effect - the shift of human color perception toward blue in low light.
- Luminaire - a complete light fixture: lamp plus everything around it.
- Kelvin (K) - the unit of color temperature. Low is warm, high is cool. Not a measure of heat or saturation.
- Lumen - total light emitted by a source. Luminous flux.
- Lux - light landing on a surface. One lumen per square meter.
- Light modifier - shade, diffuser, reflector, filter or lens. Sets the final character of the fixture.
Check yourself
- Why is film moonlight blue when moonlight is physically slightly warm?
- What is the difference between a lumen and a lux, and which one describes what lands on your wall?
- Roughly how many lux is direct daylight, an office, a corridor?
- Why is wattage a bad measure of brightness now when it used to work?
- Does a higher Kelvin number mean a hotter lamp? A more saturated one?
- Why does Dinur say the modifier matters more than the bulb, and how does that apply to a projector?