Specialty Terms

The MK350 User Manuals will explain most of the features and functions, which we will skip here (you should read the User Manuals at some point). In this section we’ll look at some of the special terms and functionality that were not covered in depth in the User Manual.

We’ve already talked about how a meter device can capture a white light and derive an x, y coordinate for our chromaticity diagram. In that discussion, we used a prism to separate the colors in white light – but actually, in our MK350 devices, we have what is called a diffraction grating that does the same thing but more efficiently and effectively.

 Diffraction Grating

The simplified explanation is that light is received through a small slit which transmits and reflects off lenses and the diffraction grating to finally reach the CMOS sensor which is the device that actually senses the dispersed light waves to be recorded as wavelength and intensity data.

Cosine Correction

Cosine is a mathematical function used to calculate and specify the characteristics (length & angles) of triangles (Trigonometry). The orange circle in the graph above was described by Johann Heinrich Lambert in 1760 and is called “Lambert’s Cosine Law”. Basically, it shows how light intensity changes as the angle of the light changes

Another term you will run into is Cosine Correction. It is associated with measuring illuminance or how much light brightens a certain area – it is measured in LUX which is lumens per sq. meter. A lumen is a unit of mea- sure that represents an “amount” of visible light.

First, you must know that light coming in at an angle will influence the brightness or LUX. A light shining from directly above will shine brighter than a light coming in at an oblique angle. In fact there is a direct cor- relation between the angle of the light and amount of brightness – this was explained by Johann Heinrich Lambert in 1760.

Measuring light intensity on a flat light meter lens is not so straight for- ward, especially when trying to measure light coming in at angles and then accurately recording the brightness according to the Lambert’s laws – you need to make adjustments or innaccuracies will occur. These adjustment are called Cosine Correction and there is a chart (next page), which is a guide

to measuring brightness according to Lambert’s laws.. The MK350 (as with most light meters) uses a Diffuser or Cosine Diffuser that “diffuses” the light to arrive at a more appropriate calculation of LUX. Diffusers are physically apparant on certain meters where you can see a white opaque material just under the lens. However, no diffuser is 100% accurate and you can only attempt to arrive at ideal corrections for light arriving at angles.

Ideally, Cosine Correction for oblique (angular) light should follow the graph shown below. For example, a light hitting our surface at 60 degrees should register radiant intensity in the meter at around 50%.  As the angle becomes less oblique, the intensity should increase according the circular orange line in the graph below. A Diffuser is in- stalled in MK350 devices to get as close to the ideal pattern (orange circle) as possible.

Dark Calibration

Dark Calibration is an optional operation in all of the MK350 devices. The devices will always ask if you want to perform a Dark Calibration each time you turn on the device. While the device is in operation, you can also, through the “Options” menu, manually execute a Dark Calibration. What is Dark Calibration used for?  Basically, an electrical residue or“charge” accumulates on the CMOS sensor and may interfere with accurate measurements. This charge builds up when the temperatures you are operating in changes significantly. The other situation is when you move from lighter to darker environments or vice versa. Also, it is a good time to perform a Dark Calibration anytime you are experiencing some anomalies in measurements. The MK350 devices always ask if you want to perform a Dark Calibration when powering on your device because it is just good practice to do so.