# Measuring "Horizontal Illuminance"

Suppose I have a light meter. And suppose I want to argue with the city that the streets in my city aren’t properly lit at night. I can find recommended valued for street lighting in lux and foot-candles. But I guess they assume that anyone with such a toy has been trained to use it. (and which unit do lighting engineers use nowadays?)

The recommended values for streets, sidewalks, etc are in horizontal illuminance. I found a formula online that involves using cosines and distance to the source and whatnot. But how do I actually take a measurement? At the street level? At eye level? Where to I point the sensor- up, sideways? Aimed at the nearest street light? Horizontally towards the nearest street light or towards the side of the street?

This is just stuff I barely remember from college, so someone correct me if I’m wrong, but:

I think horizontal illuminance is light hitting a flat surface, like the pavement. This street light vendor defines it at pavement level: http://www.acuitybrandslighting.com/sustainability/External_Light_Levels.htm#Pedestrian_Ways0

So you would take lux readings every X feet, recording where you took them, and then maybe average them out over the area in question. It’s easy to convert to foot candles if someone needs it.

That same vendor makes 3D lighting analysis software to help you estimate light levels along a roadway. The simpler, free online version is here: Visual Roadway Tool

If you choose to use that software, it’ll make your job easier if you can figure out what type of fixtures those particular street lights are and how old their bulbs are, since they degrade X% every year. Maybe do a public records request from your city? Or just ask nicely. Or just ignore the programs and use the light meter.

But you can’t just tell someone “Hey, it’s awfully dark on my street. Can you send an technician out to take a look?” It’s sad that you have to go through such a fight

Illuminance is amount of light hitting a flat surface, per surface area, per unit time. “Horizontal illuminance” is illuminance on a horizontal flat surface. It’s measured in lux (= lumens per square meter).

If you have an incident light meter, that will give a somewhat different result. An incident light meter measures light falling on a hemispherical surface (the white ball-shaped protrusion on the meter), so the reading you get is the average illuminance for all angles. For example, right before sunset, there is very little sunlight hitting a horizontal surface (i.e. low horizontal illuminance) but still a lot of light hitting a hemispherical surface.

If you’re measuring light from a single streetlight, I think you can just take the incident light meter reading and multiply it by the cosine of the zenith angle of the lamp to get the horizontal luminance - i.e. if the lamp is 30 degrees away from directly overhead, multiply by cos(30 deg).

If you’re dealing with multiple lamps and/or a lot of reflections and indirect lighting, this won’t work. But in that case, I think the incident light reading will be fairly close.

Also, our eyes are very adaptable, so there is no need for a very precise measurement. A factor of 2 difference in brightness is barely noticeable. So I’d say just use your meter and use that value as a straight approximation for horizontal illuminance.

The one I have is “cosine corrected” whatever that means.