Illuminance is the light arriving at a surface, measured in lux with a sensor laid on that surface. Luminance is the light leaving a surface toward an observer, measured in candelas per square metre with an instrument aimed at it from the observer's viewpoint. Luminance therefore depends on the surface's reflectance and finish as well as the lighting, and it is what corresponds to the sensation of brightness.

Because a driver does not see the light falling on the road, only the light the road sends back. A dark, absorbent surface under generous illuminance can present less luminance, and therefore less visibility of an obstacle, than a pale surface under weaker lighting. Specifying luminance captures the outcome that matters. It also means compliance cannot be demonstrated with a lux meter, however accurate.

It depends on the size of the target and the distance you observe from. A one-degree field suits general road and area work. Narrower fields, down to a fraction of a degree, are needed to resolve small targets such as a sign legend, a luminaire aperture or a portion of a display without averaging in the surroundings. If the field is larger than the target, the reading is diluted by whatever else falls inside it.

By measuring the luminance of the source and of the background against which it is seen, from the position and direction of the observer whose comfort is in question. Glare metrics combine those luminances with the apparent size of the source and its position in the field of view. Because the result depends on viewing geometry, the measurement position and direction must be recorded as carefully as the values themselves.

Yes, and it is a standard application. Display brightness, uniformity across the panel and contrast ratio are all luminance measurements, taken with a narrow measuring angle at a defined distance and viewing angle. For colour performance a spectroradiometer or colour meter is required alongside it, since a luminance meter reports only the photometric quantity and not the chromaticity of what it is measuring.

Only approximately, and only for a matt, diffusely reflecting surface whose reflectance you know, by combining the illuminance with that reflectance. Real surfaces are rarely perfectly diffuse, road surfaces in particular reflect strongly in preferred directions, and reflectance changes with wear, wetness and contamination. Where a standard specifies luminance, it should be measured rather than inferred.

Traceable calibration at a stated interval, typically annually, covering both the photometric response and the measuring field. Keep the optics clean and capped, since contamination on the objective scatters light and raises readings on dark targets where accuracy matters most. Check the viewfinder alignment against the measuring field periodically, because an instrument that measures beside where the operator is aiming produces confidently wrong results.