Brightness | previous | next | feedback |
Definition: a term mostly used in a qualitative way, related to the output power and beam quality of a laser; quantitatively: synonymous with luminance
The term brightness is often used in the context of lasers and laser beams, but often with a purely descriptive, non-quantitative meaning. It is also used with various quantitative meanings; this variety is frequently the origin of confusion. In particular, brightness is sometimes meant as the photometric quantity luminance, but at other times the radiometric quantity radiance (see below). The important difference is that radiometry deals with optical powers and related quantities, whereas photometry estimates the intensity of optical radiation as perceived by the eyes.
Although e.g. the US Federal Standard 1037C recommends the use of the term brightness only for non-quantitative references in the context of physiological sensations, other uses of the term have become common. In the context of laser technology, the brightness of a laser source (in a quantitative sense) is generally understood as being equivalent to its radiance, which is the total power divided by the product of the mode area in the focus and the solid angle in the far-field; the units are then e.g. W sr-1 cm-2. The rest of this article assumes this meaning, following the usual practice in this field of technology.
For a diffraction-limited beam with moderate divergence (so that the paraxial approximation is valid), the relation between beam divergence and beam radius can be used to obtain
which shows that apart from the power and beam quality, the brightness depends on the wavelength.
For a non-diffraction-limited beam, the brightness is reduced by a factor which is the product of the M2 beam quality factors for the x and y direction.
Another possible definition, disregarding the wavelength, is to take the optical power divided by the product of M2 factors. For diffraction-limited beams, this quantity is identical with the optical power, and in general it is a good measure of the maximum optical intensity of a focus which can be generated with a certain aperture and working distance, i.e., with a limited beam divergence.
Note that the brightness characterizes a laser beam as a whole, rather than being a spatially variable quality such as the intensity.
There is also the term spectral brightness for a different quantity. It can be confusing that this is actually meant by some authors when they mention only "brightness". Still another use of the term brightness refers to the perceived brightness, often just as a qualitative term, related to the luminance.
Increasing the Brightness
An important property of (optically pumped) lasers is that they can often generate laser beams with a brightness which is much higher than that of the pump source: while the output power is lower than the pump power, the beam quality can be much higher. Such a laser is then sometimes called a brightness converter, particularly if this function is an essential feature for its application.
Using Low-brightness Pump Sources
In the development of an optically pumped laser, there is often an obvious incentive to use a pump source with low brightness: such sources are generally less expensive for a given output power level. For example, broad-area laser diodes (also called high-brightness laser diodes) have a higher price per watt of output power than diode bars. However, a lower pump brightness can have various detrimental effects on laser design. For a variety of reasons, which can differ very much in different situations, the laser performance achievable in terms of output power, power efficiency, pulse duration or pulse repetition rate, is often lower in such cases.
This kind of limitations is reminiscent of entropy issues in energy conversion processes: entropy-increasing processes may not lead to a direct loss of energy, but often to an indirect loss of usable energy (exergy). Note that a brightness deterioration via a loss of beam quality basically means that radiation is spread from few modes to many modes, which indeed increases entropy. However, the mentioned performance losses are not always directly related to entropy issues, but partly related to limitations e.g. of the physical parameters of available gain media, and sometimes even to geometric issues.
See also: spectral brightness, lasers, brightness converters, beam quality, diffraction-limited beams, laser design
