Luminance

Luminance is a specific way to measure light. It is a photometric measure of luminous intensity per unit area. This measurement tracks light traveling in a specific direction. It describes how much light passes through, is emitted from, or is reflected from a particular area. This light must fall within a given solid angle. Luminance is important because it tells us how bright a surface will actually appear to a human eye.

To understand the mechanism, we must look at how light interacts with an observer. Luminance levels indicate how much luminous power a human eye can detect. This happens when looking at a surface from a specific angle of view. The solid angle of interest is the one created by the eye's pupil. When light hits a flat, diffuse surface, it can be emitted or reflected. The amount of light reaching the eye depends on this process.

There are different ways to express these measurements using various units. The official SI unit for luminance is the candela per square metre (cd/m2). You might also hear the term "nit," which is a non-SI term for the same unit. Before the SI system, scientists used the Centimetre–gram–second (CGS) system. This system used a unit called the stilb. One stilb is equal to 10,000 candela per square metre. In astronomy, scientists use a different scale called surface brightness. This uses magnitudes per square arcsecond (MPSAS).

Luminance behaves in a very specific way in geometric optics. It is considered invariant in these systems. This means that for an ideal optical system, the output luminance is the same as the input luminance. Even in real, passive optical systems, the output luminance equals the input. For example, a lens might create a smaller image of a source object. This concentrates luminous power into a smaller area. This increases the illuminance at the image. However, the light at the image plane fills a larger solid angle. Therefore, the luminance remains the same. An image can never be brighter than its source.

We can see huge differences in luminance levels across the world. A typical computer display emits a relatively low amount of light. In contrast, the sun is incredibly powerful. At noon, the sun has a luminance of about 1,600,000,000 cd/m2. Because these levels can be so high, there are important safety standards. High luminance can cause retinal damage in the human eye. This happens because of local heating of the retina. It can also cause photochemical effects, especially at short wavelengths.

To keep people safe, organizations create strict guidelines. The IEC 60825 series provides guidance on eye safety regarding lasers. Lasers are considered high luminance sources. There is also the IEC 62471 series. This series helps evaluate the photobiological safety of lamps and lamp systems. It includes things like LEDs. This standard was originally prepared as Standard CIE S 009:2002. It helps control hazards from electrically powered sources of optical radiation.

Scientists use specialized tools called luminance meters to study these values. A luminance meter is a device used in photometry. It measures luminance in a particular direction and solid angle. The simplest versions measure light in just one direction. More advanced versions are called imaging luminance meters. These work much like a digital camera. They can record luminance across an entire image. This is very helpful in the video industry. It allows experts to characterize the brightness of various displays.