technical note flicker measurement display & lighting measurement

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Christal Little
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1 technical note flicker measurement display & lighting measurement

2 Contents 1 Introduction Flicker Flicker images for LCD displays Causes of flicker Measuring high and low frequencies Introduction Low and high frequencies in Admesy instruments Flicker calculation methods Introduction General flicker calculations LCD contrast min/max method LCD contrast RMS method LCD JEITA method LCD VESA method Lighting: flicker percentage and flicker index Light sources Flicker percentage and flicker index definition

3 1 Introduction 1.1 Flicker Flicker can occur in all sources that emit light and flicker measurements are commonly used in the display and lighting industry. Flicker is a frequency domain effect and can cause discomfort in humans and animals, additionally in some cases it can lower the lifetime of a device. Some effects it can cause in humans are: Neurological problems, including epileptic seizure Headaches, fatigue Blurred vision, eyestrain Apparent slowing or stopping of motion Reduced visual task performance Distraction In an LCD, flicker is a phenomenon that is caused by Vcom offset usually resulting in a frequency of half the frame frequency. In lighting we can differentiate high frequency flicker or stroboscopic effects and low frequency flicker often caused by bad power supplies. Flicker for display measurement is defined as the visible flicker only. The calculation methods contain a form of low-pass filter. This can be one of the following: A low pass filter in hardware A low pass Butterworth filter in software A multiplication done in software on the FFT (JEITA/VESA method) Flicker needs to be measured using a human vision corrected sensor (CIE1931 luminance response) and using a correct acceptance angle. It should basically look at the light source in the same way the human eye does, otherwise the measurement result will not correlate to what we see. Currently a number of methods to calculate flicker are defined. All have their advantages and disadvantages, which are discussed in this document. 1.2 Flicker images for LCD displays In an LCD, flicker depends also on the driving technique of the display. This means some images will show flicker, while other images will not. Think about items like dot or line inversion and the different result an image will give. There are many images possible and these all depend on the display driving technique. The display manufacturer should be able to select the correct image for testing. Admesy provides some images in the LCD demo software. These are only provided as a demo and may not be most suitable for the display that is tested. This means that the display manufacturer needs to provide the correct image or at least advise how this image should look like. Note: the new IDMS ( prescribes a moving image for flicker measurement. This is not always possible for pattern generators. 1.3 Causes of flicker In an LCD, flicker is a common issue that is caused by a DC offset in the LCD and can be adjusted by changing the common voltage (Vcom). In lighting and displays one of the root causes for flicker can also be the power supply of the light source (or backlight in an LCD). Any AC signal or non-stable behaviour can cause visible flicker. 3

4 2 Measuring high and low frequencies 2.1 Introduction To measure high frequencies, a high sample rate and high pass electronics are necessary. Some instruments may have a high sample rate but are limited in electronics regarding the maximum frequency (e.g. a hardware low pass filter). To measure low frequencies, the measurement needs to cover a long time to capture multiple frames of the low frequency. 2.1 Low and high frequencies in Admesy instruments Every Admesy instrument that can measure flicker has two parameters for capturing the signal: Number of samples Delay The sample rate of each instrument is different, but the use of the parameters is the same. When the number of samples is increased, the measurement time will increase. When delay is increased also the total measurement time will increase. Example1: When we are interested in measuring only higher frequencies, we have no need for the delay function so we set it to 0. We chose the number of samples such that the measurement becomes stable. Example 2: When we want to measure low frequencies, for example 1Hz, than we don t need a very high amount of samples, but we need the delay to increase the measurement time. For example we take 2000 samples and set the delay to 50. Example 3: Measuring both high and low frequencies can be done by combining the settings. For example with a Cronus, you could set the number of samples to 100,000 and the delay to 20. This would measure a total time of about 8 seconds. 3 Flicker calculation methods 3.1 Introduction At present, the calculation methods for flicker differ between the display industry and lighting industry. Additionally, multiple non-official calculation methods exist. Admesy currently supports the following standards for display measurement: Contrast min/max method Contrast RMS method JEITA method VESA method Admesy supports the following calculations for lighting measurement: Flicker percentage Flicker index 4

3.2 General flicker calculations In general, flicker can be defined as the ratio of the AC level of the signal and the DC level of the signal. AC level DC level Fig 1 AC and DC levels of signal.

The next chapters cover existing flicker calculation methods used in the display and lighting industry in more detail.

5 3.2 General flicker calculations In general, flicker can be defined as the ratio of the AC level of the signal and the DC level of the signal. AC level DC level Fig 1 AC and DC levels of signal. flicker = AC level DC level Equation 1 This is a simplified model of course. The AC component can be calculated in various ways. Therefore multiple definitions exist for flicker measurement. The next chapters cover existing flicker calculation methods used in the display and lighting industry in more detail. All Admesy instruments can output the raw signal, so that new calculation methods can always be added by implementing them in the PC software. Additionally some devices support the currently defined calculations directly from firmware using simple measure:flicker commands. 5

6 3.3 LCD contrast min/max method This method calculates the AC level by taking the maximum and minimum of the signal. The DC level is defined as the average value of the signal. Min Max DC Fig 2 AC and DC levels of signal. flicker = (max min ) average 100% Equation 2 flicker = 10log 10 (max min ) average db Equation 3 Advantages Simple: Always possible to embed in firmware of a device. Disadvantages Requires a low pass filter either in hardware or software to filter higher frequencies. 6

7 3.4 LCD contrast RMS method The LCD contrast RMS calculation method calculates the AC value of the signal as the RMS (Root Mean Square) value of the signal. The DC level is defined as the average of the signal. RMS DC Fig 3 RMS and DC levels of signal. flicker perc = Equation 4 x DC equals average value of the signal. 1 n n 1 i=0 (x i x DC ) 2 100% x DC flicker db = 10log 10 ( flicker perc ) 100 Equation 5 x DC equals average value of the signal. Advantages Reasonably Fast (but slower than min/max method) Reasonably Simple: Always possible to embed in firmware of a device. Disadvantages Needs a low pass filter either in hardware or software to filter higher frequencies. 7

DC Lowest detected frequency Fig 4 RMS and DC levels of signal. DC level P0 = 500 Lowest detected frequency P1 = 30 Fig 5 FFT.

8 3.5 LCD JEITA method The JEITA method is based on frequency domain calculations. It uses an FFT to determine the AC and DC level of the measured signal and translates the signal into an FFT (figure 5). DC Lowest detected frequency Fig 4 RMS and DC levels of signal. DC level P0 = 500 Lowest detected frequency P1 = 30 Fig 5 FFT. After calculating the FFT, a weighting factor is applied to compensate for human eye sensitivity to frequency. This has been defined as shown in the graph (fig 6) and table below (table 1). Fig 6 8

9 Frequency Factor (Hz) db Ratio Table 1 This means that in the signal we measured, P1 at 30Hz should be reduced by -3dB. The 60Hz component should be reduced by -40dB but since it is lower than the 30Hz component, it is not used in the final calculation anyway. The amplitudes found on the previous page (P0 and P1) are reduced by the factors and the final result will then be: Equation 6 P r 0 = P0 1 Equation 7 P r 1 = P Equation 8 flicker JEITA = 20log 10 ( P r1 P r 0 ) db Advantages No need for an additional low pass filter. Disadvantages Slowest method, on embedded systems sometimes too slow to be useful. Needs a fast CPU for FFT calculation. In case of only a few number of samples, it is not always the most stable method. Due to relatively low speed it is not suitable for production settings. 3.6 LCD VESA method The VESA method is equal to the JEITA method with a small difference in the result calculation which results in a difference of approximately 3.01dB caused by squaring the amplitudes in the FFT. Equation 9 flicker VESA = (flicker JEITA + 20 Log 10 ( 2) )db 9

The power supply can be a root cause for flicker and the faster a light source responds to power changes, the more this effect will be recognized.

10 4 Lighting: flicker percentage and flicker index 4.1 Light sources In lighting, the power that is used is either directly from the mains power or through a power supply. The power supply can be a root cause for flicker and the faster a light source responds to power changes, the more this effect will be recognized. With fluorescent light sources or incandescent lamps the effect is reduced due to the slow response of the lamp itself, but still this doesn t mean that these lamps have no flicker. Fig 7 Incandescent lamp: flicker percentage 6,6%, flicker index 0,02. Fig 8 Magnetically ballasted-fluorescent tube: flicker percentage 28,4%, flicker index 0,07. 10

Fig 9 CFL lamp: flicker percentage 5,1%, flicker

Fig 10 LED lamp: flicker percentage 54,7%,

When looking at LED lighting, the problem is a

11 Fig 9 CFL lamp: flicker percentage 5,1%, flicker index 0,01. Fig 10 LED lamp: flicker percentage 54,7%, flicker index 0,17. When looking at LED lighting, the problem is a lot bigger because the LED responds very fast to changes of the power supply. Ideally the LED is driven by a DC current, but this is not available from the main power source, so requires expensive electronics components. 11

12 4.2 Percentage flicker and flicker index definition The advantage of percent flicker is that it uses a 0-100% scale. It is also referred to as peak-to-peak contrast or Michelson contrast in literature. The advantage of flicker index is that it uses a scale. It is less well known at this stage and not used as often as the percent flicker method. Both methods are based on the analysis of one cycle of the periodic waveform and does not account for any frequency component, which is the biggest difference form the LCD flicker measurement methods. The following graphical representation shows the way of calculating percent flicker and the flicker index based on capturing one cycle of the optical signal. Maximum value area 1 Minimum value Average area 2 light output Fig 11 Example of one light fluctuating cycle and relations of flicker. Equation 10 percentage flicker = 100 max min max + min Equation 11 flicker index = area 1 area 1 + area 2 12

13 area 1 area 2 flicker index = 0.25 flicker percentage = 50 Fig 12 Flicker index and percentage example. 13

14 Admesy B.V. Sleestraat CA Ittervoort The Netherlands T +31 (0) F +31 (0) info@admesy.com The material in this document is subject to change. No rights can be derived from the content of this document. All rights reserved. No part of this document may be reproduced, stored in a database or retrieval system, or published in any form or way, electronically, mechanically, by print, photo print, microfilm or any other means without prior written permission from the publisher. Version /

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