If you ILLUMINATE an object and measured the amount of light being projected towards it by the light, that is ILLUMINANCE.

LUMINANCE is the amount of light reflected off the surface being ILLUMINATED!

Knowing the difference between Luminance and Illuminance is about remembering what has happened to the beam of light; the action of light hitting a surface – a beam of light that is travelling towards a surface – is what is measured when talking about illuminance.

Illuminance is measured as the amount of light striking a surface. If we had a light meter and wanted to measure the amount of light that was striking the surface we were measuring, we would hold it towards the light.

Luminance, is what we measure off of the surface that has light hitting it. Luminance is the measurement of the product of the incident light and the surface – anything that is reflected. Luminance is also considered the human perception of brightness, or how bright we perceive the light that is reflected off of the surface.

Make sense? Think of it like this – IL-Luminance, IL, I = Incident Light. Illuminance is measuring the incident light. Luminance is what’s leaving the surface – L = leaving. Illuminance is measuring incident, luminance is measuring what’s leaving.

You must understand the difference between luminance and illuminance if you want to gain a deeper understanding of the lighting world. Most people that don’t know about these terms refer to displays in terms of brightness. Yet, it’s crucial to know that luminance and illuminance aren’t synonymous with brightness. You can’t measure brightness; it’s something perceived by the human eye. Since both luminance and illuminance are quantifiable terms measured using specific tools, you’d be incorrect to use them interchangeably with brightness.