Radiometry and Photometry

Lighting terms can be rather technical and confusing. This may help.

Radiometry is the science of the measurement of electromagnetic (EM) radiation. The broader spectrum covered by the science of radiometry is based on physical constants.

The properties of concern to us here are radiated power and its spatial and angular distributions. The four basic concepts are:

1. Radiant Flux
2. Radiant Intensity
3. Radiance
4. Irradiance

Photometry is defined as the measurement of electromagnetic energy that is visible to the human eye.

In 1942 the Commission Internationale de l’Eclairage (CIE) convened to define the response of the average human eye. They defined the Standard Luminosity Function also known as the Photopic curve and the Scotopic curve.

Photometric quantities are derived from radiometric quantities by utilizing a filter which approximates the behavior of the average human eye as defined by the CIE.

A radiometer is a device that measures the intensity of radiant energy. A majority of radiometers utilize only a single sensor whether photocell, photo-multiplier tube (PMT), etc. An optical filter is normally used when measuring radiation emitted from a specific spectrum or to incorporate the radiometer within a certain spectral response.

Radiometers are commonly used to quantify light or energy outside the visible spectrum, i.e., ultraviolet and infrared. Depending on the specific application, different filters with specific spectral responses are used.

A photometer is defined as an instrument for the measurement of visible light. The most common types are the luminance and illuminance meters.

The basic difference between photometers and radiometers is that the photometer incorporates a filter which follows the CIE Standard Luminosity Function Vλ curve (shown below).

Konica Minolta Sensing Inc published a booklet - The Language of Light which helps make the complicated lighting terminology simple for applicators.

Write to me if you need a copy.

Crusing 100 Miles Per Hour

The alarm sounded and I opened my eyes and noticed that it was 5 am. Everything seems dark and I turned on the lights and my eyes instantaneously close as the bright light glare. It felt the same last night as I slipped into bed and the minute I turned off the light, I need to close my eyes before getting use to the dim surrounding.

This phenomenal is due to the design of our eyes. Our eyes have two types of light-sensing cells called rods and cones.

The cones are responsible for bright light vision and the rods for night time vision. Cones have a peak response at about 555 nano-meters and rods have a peak response in about 505 nano-meters. Because both the rods and cones have different sensitivities, they are represented by two different sensitivity curves called photopic curves (representing the cone) and scotopic curves (representing the rod).

Photometry involves measurement of the psychophysical attributes of electromagnetic energy that is visible to the human eye. Photometry is the science of human perception.

Now imagine that you are driving into a tunnel. Bright and sunny highway into a dark tunnel. And your eyes sensors switches from photopic to scotopic. Your eyes shut tightly as you adjust to the dark condition for a second or two. And if you do continue your journey, leaving the dark tunnel into the bright highway ....

This is a major concern for highway engineers. The next time you drive into and out of a tunnel, notice the lighting into and leaving the tunnel, they are specially built to make sure you don't close your eyes. It is not about the lights only, there are instrumentation to make sure that lighting levels are balanced according to the time of the day (or night).

Happy cruising.

Photometry becomes a modern science in 1942, when Commission Internationale de l'Eclairage (CIE) metto define the response of the average human eye. CIE measured the light-adapted eyes of a sizeable sample group, and compile the data into the CIE Standard Luminosity Function (widely known as photopic curve -chromatic perception at normal state, and scotopic curve - achromatic perception at low level of illuminance.