Brightness and luminance

"Brightness" is used in three senses, and a specification should say which. In vision it is a perception, how bright something looks, which depends on adaptation and surroundings and is not directly measurable. In photometry its measurable counterpart is luminance. In laser physics and optical engineering, brightness usually means radiance, the power per unit area per unit solid angle, and a "high-brightness" diode laser is one that delivers its power from a small area into a narrow angle.

Photometric units

Photometric quantities weight the radiant power at each wavelength by the eye's spectral sensitivity for daytime vision, $V(\lambda)$, which peaks at 555 nm, and multiply by 683 lm/W. That constant is fixed by the SI definition of the candela at 540 THz, essentially 555 nm, so 1 W at 540 THz (about 555 nm) is exactly 683 lumens, while 1 W of 1550 nm light is zero lumens, however bright it is to a detector. The main quantities pair with radiometric ones:

Luminance is the one that corresponds to how bright a surface looks and is conserved, like radiance, by passive optics; displays, projectors and illumination standards are specified by it.

Why lasers are "bright"

A laser's brightness in the radiance sense can exceed the Sun's by orders of magnitude at milliwatt powers, because the light leaves a small area into a tiny solid angle; the radiance entry works through a helium-neon example. For high-power lasers the equivalent figure is the beam parameter product, and "brightness" in a diode or fiber-laser datasheet often means power divided by the square of the BPP. Since étendue cannot be reduced passively, brightness sets how tightly a source can be focused and how efficiently it couples into a fiber.

Measurement

Luminance is measured with a luminance meter or imaging photometer, a detector filtered to match $V(\lambda)$ and viewing a defined field; illuminance with a lux meter on a cosine-corrected diffuser. For narrowband sources, such as lasers and LEDs, filter mismatch errors are large, and it is more accurate to measure the spectral radiance with a spectroradiometer and weight it numerically by $V(\lambda)$.

References: BIPM, The International System of Units (SI brochure), 9th ed. (2019), definition of the candela; CIE S 017:2020, ILV: International Lighting Vocabulary; ISO 23539:2005 (CIE S 010/E:2004), Photometry: The CIE system of physical photometry.