Re: The THX 35FtL Standard

Re: The THX 35FtL Standard

Home Forums General Chit Chat and Other Stuff The THX 35FtL Standard Re: The THX 35FtL Standard


@BuzzSchranz 3642 wrote:

THX changed the standard from 30 to 35FtL because of customer complaints if I remember THX training correctly, but where did 30FtL originate and why?

Doesn’t this have more to with screen size, viewing distance, and total light? My 32″ LG LCD viewed from 8 feet looks good at 35FtL as does my 51″ Samsung Plasma viewed on average from 13 feet. However, my 75″ Mitsubishi LaserVue is outputting 21FtL and at a 10 foot viewing distance I would not want more. Then there is the standard for 14-16FtL with front projectors and in an appropriately darkened environment this too is plenty of light.

Should there be a chart somewhat like the screen size/viewing distance charts?


One of the more seminal recommended practice documents for video display calibration is SMPTE RP167 ‘Alignment Of NTSC Color Picture Monitors.’ Its companion is SMPTE RP166 ‘Critical Viewing Conditions For Evaluation Of Color Television Pictures.’ Both were the fruit of the SMPTE Professional Monitor Working Group chaired by Joe Kane back in the mid to late 1980s. Another significant result from that research was the first calibration tutorial and test program on optical disc: ‘A Video Standard’ by Joe Kane Productions on laserdisc. Human factors studies are a foundation of the standards, engineering guidelines, and recommended practices published by SMPTE.

The ideal recommended peak screen brightness for critical viewing of video monitors was stated as 35 fL (but no lower than 30 fL), from a 100 IRE window pattern. This was also understood to be in a dark room with “dim surround,” an ambient light level behind the monitor at 10% of peak white. The 35 fL brightness level was considered sufficient light output for critical analysis, extended viewing sessions, and generally attainable within the linear operating range of popular professional NTSC monitors of the day. This recommendation had little to do with viewing distance or screen size. The dominant factors were viewing comfort and linear peak luminance limitations of the CRT technology.

THX, Ltd. is not a motion imaging industry standards body. They are a consulting and research organization that helps professionals and consumers successfully benefit from and implement the work of SMPTE, ITU, ATSC, EBU, CIE, ANSI, ISO, etc., etc. For more detail and history on these topics, see: .

Best regards and beautiful pictures,
Alan Brown, President
CinemaQuest, Inc.
A Lion AV Consultants affiliate

“Advancing the art and science of electronic imaging”