Tech Thursday – Light Part 2 – Luminous Intensity and its measurement

This is the second instalment of a short three part series discussing Light.

  1. In the first part we briefly looked at what we define light to be went on to discuss some of the photometric units that are typically found in photometric measurements.
  2. In this second part continue on to discuss Luminous Intensity and in particular how we go about measuring it in a meaningful way.
  3. In the final part we round-up the series with a brief introduction to colour, its measurement and the typical colour definitions used in signalling.

Luminous Intensity

As discussed in part 1, Luminous Intensity is a measure of the light emitted in a given, or defined, direction.  For a static (non-flashing) light source it is very simple to measure the light in the given direction and to then quote a meaningful figure for luminous intensity.  Further, with the use of a goniophotometer it is fairly straightforward to measure the intensity in a number of directions and produce a 3d map of the resulting data.

Things get complicated as soon as we start to look at flashing light sources such as warning beacons (also known as visual alarm devices or VADs) as the relatively slow response of the human eye to pulsed light plays a large part in dictating how bright the flashing light appears.

Peak vs. Effective Luminous Intensity

Peak Luminous Intensity, along with Effective Luminous Intensity below, is typically quoted for flashing (non-steady) warning beacons.  As it is usually quoted as the maximum luminous intensity of a single flash, typically in whatever direction yields the greatest figure, an impressive looking figure is available for marketing datasheets.  The problem is that quoting just the maximum measurement of the short duration flashes from a warning beacon fails to account for how the eye perceives the flash – Our experience has shown us that a very short but very high intensity flash may actually appear less bright than a lower intensity, but longer flash due to how we see flashing lights and is therefore meaningless when comparing products or even products to specifications.

Effective Luminous Intensity recognises the issues with Peak Luminous Intensity above and attempts to standardise a meaningful measurement.  Effective Luminous Intensity as defined by E. Allard is the “luminous intensity (cd) of a steady light, of the same spectral distribution as the flashing light, which would have the same luminous range as the flashing light under identical conditions of observation”.  In other words, how bright does a flashing light appear to be compared to a similar non-flashing light.  This necessarily takes into account the flash-rate, flash-pattern and individual pulse shapes and provides a figure that is easily compared across different types of light source.  To further complicate matters, there are numerous methods for calculating the Effective Luminous Intensity that may or may not yield the same numerical answer depending on the exact flash-rate, flash-pattern and individual pulse shape of the light being measured!  There is a great paper on this subject written by Yoshi Ohno of NIST that can be downloaded from here.

Measurement

The measurement process of the Peak Luminous Intensity is relatively straightforward.  We have in the past connected a standard Lux meter to an oscilloscope and used this to capture a plot of the flash event.  Quoting the peak luminous intensity is as simple as reading off the maximum value and scaling the result to candelas.

We measured the Effective Luminous Intensity in a similar fashion but, rather than just reading off the result we apply the measurement, along with an estimation of the area bounded by the flash event, to one of the standard formulae and arrive at a figure that allows us to compare the luminous intensity across a range of our products and product technologies.

In more recent times we have automated these measurements by using a closed loop system that captures the flash event waveform and performs the calculations directly on the raw data.  One of the benefits is that we can directly compare the output of different integration methods to see what effect these have on the result.  This is a much faster process, that combined with a goniometer, provides a fast method of measuring the effective luminous intensity of a product in 3d space.

Summary

This is the second instalment of a three part series discussing various properties of light.  We very briefly looked at Luminous Intensity and how it relates to visual warning signals, the difference between Peak & Effective Luminous Intensity and why it matters and how we go about measuring the Luminous Intensity of warning signals.

In the final instalment we will be looking at:

  • Colour
  • Measurement of colour
  • The typical colour definitions used in signalling