Hello, everyone, welcome back. In my last posting, we examined the various scientific (SI) units used to measure many of the properties of light; in reality, most of those related to brightness or luminance. We needed this data in order to make sense of the various specifications that we come across in our daily lives as media professionals, but in particular I wanted to define exactly what is meant by the term “nit” (a colloquialism for “candela per square meter,” if you recall). The reason for this is that specification for brightness in today’s monitors is most often described in terms of nits.

As is often the case, some of the parameters of a transmitted signal were actually defined by dated technology. For example, we still have drop frame timecode, which arose from the creation of the NTSC color television standard, even though we have moved on significantly since the technology which mandated such a complex solution. That historical perspective is also responsible for what we would now consider to be “Standard Dynamic Range.”

THE UPPER LIMITS

To date, moving images distributed to the various platforms has been constrained to an upper limit of 100 nits. That is because 100 nits is approximately the limit of illumination that was achievable from a standard CRT display. Try buying a new, CRT based, television set today!

Despite the obsolescence of CRT as the ubiquitous display technology, the (SDR) programs are still calibrated to fit in this limited range. In order to be able to do that, the extremes of the signal – those close to black and close to white – end up being crushed down and up respectively in order to maximize the linearity of the end-to-end system in its midrange.

[Read: HDR: What Is It And Why Do We Need It?]

Modern flat panel displays, on average, offer a maximum light output of 500 nits, and manufacturers are already offering new sets that boast 1,000 nits. But—and here’s the important part–the majority of the video that will be delivered to them is SDR and has been conformed to fit into a 0 - 100 nit range. Thus, most of the fine detail in those extremes is lost forever. The images, when captured, are actually already raw HDR, but that dynamic range is crushed at several stages of the production and delivery process (Fig. 1).

QUALITY IN, QUALITY OUT

There is a very important point here – once that detail has been crushed, you can’t get it back. So even if you display it on a 500-nit screen, you are not getting HDR. Let me be perfectly clear: HDR does not just mean brighter whites. Setting the white point of an SDR image to 500 nits on your screen does not gain you extra detail. Think about this in audio terms: you can put an MP3 signal into a very powerful, very expensive high-fidelity audio system and crank up the volume, but the quality of the audio doesn’t get any better – it just gets louder!

In order to provide you with a higher fidelity (i.e. HDR) image on your screen, the delivery system has to possess a higher dynamic range from the point of ingest all the way to the point of display. (Fig. 2 acknowledges that we live in the real world; there is a practical limit to how much resolution – spatial (pixel count), temporal (frame rate) and HDR/WCG we can deliver to the mass audience within reasonable economic restraints. HDR is an attempt to deliver the most faithful imagery we can currently deliver, for the greatest impact on the customer experience.)

We must acknowledge that HDR is currently in its infancy. There are compromises in each of the display technologies available today. LCD screens are transmissive – they require a backlight in order to produce a viewable image. You can certainly increase the brightness of the backlight to produce white levels in excess of 1000 nits, but LCDs have a fixed contrast ratio internally, so increasing the level of the backlight also increases the light level produced by “black” pixels.

Manufacturers have come up with several schemes to modulate the backlighting in areas which are supposed to be black, with varying degrees of success. OLED screens are emissive, and do have the extended dynamic range capabilities, but struggle to produce the higher light levels, especially over long periods of time.

This last point is taken into account in all of the proposed HDR standards – metadata provided with the signal lets the set know just how bright the average image is and will also specify just how bright the brightest pixel in the entire program is, so that a display can adjust its behavior in order to protect itself from premature burn-out.

We will discuss these proposed standards in a future posting, but first we’ll examine just what is meant by “Wide Color Gamut” and discuss the various standards currently in use in both standard dynamic range HD (i.e. today’s signals) and in the proposed HDR formats.

Paul Turner is founder ofTurner Media Consultingand can be reached atpault@turnerconsulting.tv. 

In my last installment, I discussed some of the science behind how the human eye actually perceives light—important to know as we continue to strive to deliver to the viewer the highest fidelity reproduction of the scene they would see if they were actually there. Part of that evolution is to produce images that cover a much higher range of (and finer granularity of) light delivered by the viewing screen—that’s one of the tenets of HDR. Again, though, I must warn the reader that HDR is not simply about delivering brighter pictures—you can do that by turning the brightness waaaay up on a modern screen. That doesn’t give you HDR, as it does nothing to improve the reproductions of shades down in the black area.

To improve on light delivery, you first need to be able to measure its intensity. Once again, the video engineer finds him/herself with some of the information, but as in all new technologies, there are endless acronyms and SI units that make up the discussion, and it’s very easy to mix them up when learning about that new technology. In this blog, I’ll shed some light (sorry—had to get that pun in somewhere) on how these various units of measurement relate to each other —so you will be able to answer the question “how many Candelas is a Nit?” and wow your friends at cocktail parties!

WHAT IS A STERADIAN?

First, though, we need to familiarize ourselves with the “steradian,” because that term is thrown around like spaghetti when discussing light and illumination. The steradian is the SI unit of a solid angle, which in turn is the ratio of the area subtended on a sphere by that angle and the square of the radius of the sphere. Confused? Fig. 1 makes it clearer, I hope.

The basic unit when talking about luminous intensity is the candela, often abbreviated to “cd” and is basically a measurement of luminous power per unit solid angle. Basically, it is more or less the luminous intensity emitted by a common candle. That, of course is not accurate enough for scientific measurement, but it will suffice for a mental picture. The scientific definition is “The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540×10 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.” I like my simple analogy better!!!

So now we have to consider the measurement of the perceived power (by the human eye) of light. This quantity is called “luminous flux” and is analogous to “radiant flux”—the measurement of the total power of electromagnetic radiation—except that it takes into account the eye’s variance in sensitivity to different wavelengths of light. In other words, it doesn’t include the infrared or ultraviolet spectra, and other wavelengths are weighted by the eye’s sensitivity. Luminous flux is measured in lumens (lm). 1 lumen is defined as 1 cd multiplied by the steradian of the beam for which we’re making the measurement (a sphere has a solid angle of 4P steradians, so a source delivering a uniform 1cd in all directions is emitting 12.57 lumens). Why do you care? Because most light sources are commonly labelled with their output in terms of lumens.

WHAT IS A LUX?

The next term in our “dictionary of light” is the lux (lx). This one is very simple: it defines how much luminous flux is spread over a given area. 1 lux is simply 1 lumen per square meter. If you had the same 1 lumen over an area of 2 square meters, you would only have 0.5 lux and it would appear half as bright. Pretty intuitive.

Why do you care? Because if you try to look up the light levels for bright daylight or the moonless night sky, you’ll most likely get values in lux (approximately 25,000 lx and 0.002 lx respectively).

WHAT IS A NIT?

And finally, we come to the final metric that we need to understand: Nits! The scientific name for this unit is “candela per square meter” (cd/m), but for some reason, the term “Nit” (nt) has become widely adopted when talking about displays (or screens)—the two terms are interchangeable. To give you a sense of perspective, a sunny day will measure somewhere around 10,000–12,000 nits and a moonlit night, with the moon directly overhead, will measure in the very low single (or even fractional) digits.

So there you have it—the fascinating world of light measurement. Note that we’ve only discussed measurement of total intensity here, not colorimetry. We’ll be looking at how all of this impacts HDR in our next discussion, where we’ll also talk about colorimetry and wide color gamut. As I mentioned earlier, HDR and WCG are actually different topics, but they are almost always spoken of together when discussing HDR in general. See you next time!

Paul Turner is founder ofTurner Media Consultingand can be reached atpault@turnerconsulting.tv