Preface
The international multi-sport Olympics held in Rio de Janeiro, Brazil is just finished this summer. Each country has corresponding broadcaster to present high quality sport programs to supporters who unfortunately was not able to watch the game on site but sit in front of TV. One of successful keys providing high quality sport program is the slow motion replay. Slow motion replay can provide dedicated movement of athletes and sometimes it could be used for judges to make a precise and fair decision. The conventional video speed is 24 fps (frame per second) for NTSC system and 25 fps for PAL/SECAM system. The slow motion is achieved by increasing the fps. Each captured frame or image needs to have equal light to avoid any uncomfortable visual effect or simply called flickering shown in the video. Therefore providing a constant light from artificial light source is the key to success for slow motion capturing. The amount of flickering is determined by the parameter so called Flicker factor or Percent flicker. This article explains why Flicker factor affects the performance of slow motion and how MEAN WELL’s LED driver can fulfill the requirement.


Figure 1
Illustration of Current ripple and Percent flicker or Flicker factor for stadium applications
where Imax , Imin is the maximum and minimum output current respectively;
EHmax , EHmin is the measured maximum and minimum illuminance in time

 
Current ripple and Flicker factor
The traditional lighting system for sport stadium is based on metal halide light (HID) which consumes lots of electrical energy. Therefore more and more stadium is replacing HID with LED lighting system. Except for energy saving, other advantages of using LED lighting in stadium include wider dimming range, uniformly distributed light, better color rendering (CRI>80), lower stray light, no waiting time when re-start the system, longer life time, no or low flickering for slow motion video capturing. The physical operating mechanism of LED is to convert the electrical energy from LED driver into the light. If there is a variation in the electrical energy, similar variation (not same but luckily smaller) will be presented in the output light format. Such variable light results the unequal light in each frame when in slow motion. As result LED driver which determines the level of electrical energy variation is the key component to judge the flickering performance of a LED lighting system. The ideal output current of a LED driver should be flat and stable over time. The real output current shows a repetitive variation over a fixed period and such parameter is called Current ripple with formula shown in the Figure 1. The Flicker factor or Percent flicker parameter is used to evaluate the variation or flickering performance from an artificial light. The designer of LED lighting system can just take the Current ripple from driver to quickly evaluate the flickering performance because the Flicker factor result of light would be similar and slightly better than Current ripple of driver.

Slow motion requirement for stadium
The averaged Flicker factor of lighting system should be less than 5% for Elite Level A stadium under 300 fps slow motion video capturing according to the Union Européenne de Football Association (UEFA) stadium lighting guide 2016. The Level A and B stadium should have averaged Flicker factor less than 12% where the Level C stadium should have averaged Flicker factor less than 20%.

The Current ripple of LED driver
The Current ripple parameter can be found in the datasheet of constant current LED driver. For instance, MEAN WELL HLG-320H-C series has Current ripple 5%. The actual measured Current ripple can refer to the online test report. As shown in the Figure 2, the measured Current ripple is 1,14% which meets the 5% requirement for Elite Level A stadium in UEFA.


Figure 2 Current ripple from specification of MEAN WELL HLG-320H-C series (Left);
The measured Current ripple from test report of HLG-320H-C1400 with output current 1400 mA (Right)


Summary 
The slow motion video capturing is very depending on the artificial light. The driver is the key component to determine the flickering performance in a LED lighting system. Depending on the purpose and requirement, MEAN WELL offers a complete product line and technical support such that customer can quickly build up a LED lighting system suitable for slow motion capturing applications.

The author: Wen Wu Email: [email protected]