What is luminance ratio?

Luminance ratio is a term used for the useful contrast of a display, the actually perceived contrast in the context of the room it is installed in. This is the contrast ratio including the ambient light which is reflected on the display. The actual contrast you perceive as a user will not only depend on the display specifications but also on the conditions in which the display is installed.

A highly recommended read: The following knowledge base article explains in a comprehensive way what ambient light is and how we compensate for it. You can find it here: [KB7842]

What is the difference with contrast ratio?

Contrast ratio defines the theoretical contrast you can get with your display, however, this contrast ratio can only be achieved in optimal conditions which cannot be reproduced during regular use.

How do I calculate the luminance ratio?

You can use the following easy formula:

LR = L'max / L'min

Where:

• LR = Luminance ratio
• L'max = The display's brightness + the reflected ambient light.
• L'min = The display's black brightness + the reflected ambient light.

When you take a good look at this formula, you see that you divide by L'min. Lmin is typically a rather low number (e.g. 0.4 cd/m²). When you start adding the reflected ambient light to this (to get L'min), you can easily double this value which means that (reflected) ambient light can have a huge influence on the luminance ratio.

An example:

You have a display with the following specifications:

• Lmax = 600 cd/m²
• Lmin = 0.5 cd/m²

The reflected ambient light on the display, which I can find back in QAWeb, is for instance 0.2 cd/m² (which indicates that there is very little light in the room).

So that means that we have the following luminance ratio calculation:

LR = (600+0.2)/(0.5+0.2) = 857.43

This is an excellent luminance ratio considering that current QA standards and guidelines recommend a luminance ratio of 350 or higher.

However, when you turn up the light in the room and the reflected ambient light goes up dramatically, for instance to 0.8 cd/m², take a look at the calculation:

LR = (600+0.8)/(0.5+0.8) = 462.15

As you can see, the luminance ratio went down quite quickly.

How can I measure the luminance ratio?

Well, you cannot actually measure luminance ratio with one measurement but you can measure L'max and L'min rather than using the QAWeb calculated reflected ambient light.

Prerequisites:

• A workstation with QAWeb Agent installed and the displays connected.
• An external luminance sensor which supports distance measurement. (Find out what the difference is between a distance measurement and an on-screen measurement in the following article: [KB7954]

Use the QAWeb Agent test patterns 'AAPM TG18-LN01' (for L'min) and 'AAPM TG18-LN18' (for L'max). Take care to hold your sensor perpendicular to the display, in the center of the test pattern and at the sensor vendor recommended distance.

Conclusion:

This article and the examples above should illustrate the importance of knowing how ambient light influences the display's characteristics.

A few tips for achieving a good luminance ratio on your medical display:

• Take care to purchase a display which has enough luminance.
• Make sure that the lighting in the reading room is not too bright to ensure a good luminance ratio.
• Keep your ambient light stable to make sure your luminance ratio remains stable over time and never drops below the legal threshold (if relevant in your region).