The Importance of Contrast in LED Signage
A conversation on image quailty and contrast in LED displays, why it’s important and how to test for it.
Justin Ochsner on 3/27/2020
Recently, Matt and I sat down with Shannon Mutschelknaus, engineering manager of Daktronics laboratory services group, for a conversation on The Daktronics Experience podcast. We discussed image quality and contrast ratios, why contrast is important, how we test for it, and much more. Here are some of the main points from our conversation. Justin: Shannon, can you give us a definition of contrast ratio and how that plays into image quality? Shannon: Contrast ratio is a great topic with a lot of confusion out there. Contrast ratio starts off simple. It’s the ratio of the light that your display puts out when it’s on to when it’s off, when it’s displaying a black image to when it’s displaying a white image. It is the ratio of that. Matt: I’m curious as to the origin of it. There’s a lot of confusion out there. I immediately just jump to seeing it in different ads and seeing it in different spec sheets, but where did all this start? Shannon: I think some of the confusion comes from there being a lot of different types of contrast ratio and a lot of different ways to measure and characterize it. Some of them are really hard to do, and some of them are really easy to do. So, we have a lot of different metrics. Sometimes we just say it’s the contrast ratio, but there are a lot of different kinds of contrast ratio. We don’t distinguish that well in spec sheets or when we are talking. Justin: Does the environment that it’s measured in also factor into this? Shannon: Yes, sometimes on a display the light output is pretty dynamic, so different things can cause it to be different light output. The basic contrast ratio, what usually you are seeing on spec sheets when you are buying a tv or cell phone in the store, is in a controlled environment, a black room or dark room with no lights below a minimum light level. When you send a black image or a white image to the display, you measure the lumens. The light output or NITS. We often don’t realize that the contrast ratio we are seeing on the spec sheet, that’s in a dark room. That doesn’t have a very great relationship to what you actually observe in normal usage conditions. I think we probably all experience that with our cell phones. This is South Dakota. We are out pheasant hunting. We get a phone call, we try to answer our phone. Normally, if I was in town, I’d go walk under a tree with the shade so I could see my screen. But with no trees, we try to read our phone and, “Oh, I can’t see what my screen says.” That’s the contrast ratio. That’s actually what we call the ambient light contrast ratio. Sometimes called ACR. Contrast ratio measurement has been a key thing for a long time. In your display, a lot of people would say this is the most important aspect or property of the display to distinguish your image quality. Because it’s the ratio of light to dark. It’s what you can see. So, you need a good contrast ratio or you can’t distinguish the images. Justin: So, it’s like if you’ve got somebody wearing a certain color shirt, say it’s a blue shirt on the display, and there is the blue sky background but they are different shades of blue. Your contrast ratio is going to set those two apart? Shannon: Absolutely. That’s even another kind of metric. What I was talking about is just sending a black and a white image. You also have color contrast, which is another type of ratio that isn’t usually communicated. It’s also really important. Justin: Yeah, the display has to have enough contrast for you to read it. But also, the size of the contrast ratio affects how fast the viewer can read it. So, that is a really big thing with our DOT signage. They need a really big contrast ratio because you want the human response time, when they see that, to be fast. Shannon: Absolutely. It’s really important. Contrast ratio has been measured for a long time and it can be confusing. So, let’s start with the basic one, in a dark room. Are you guys old enough to remember CRT’s? Matt and Justin: Yeah. Shannon: So, CRT’s, they can’t show a true black. Even when you send a black image to it, they emit some amount of light. Sometimes, maybe you have witnessed that at bedtime you turn it off and there is still a bit of a glow. That’s kind of the lowest level of black that they can put out. It cannot get to an absolute zero. LCDs are even worse than that. They have a backlight behind there that is always on. Well, it used to always be on. Now there are different kinds of backlights, like the LED backlights, but they can dynamically turn those LEDs off and on. But an LCD operates with a light behind it shining through a filter that the crystal turns on and off. But you always have light leakage through that display even when you send a black image to it. Justin: There is a phrase out there that contrast ratio is the most important but the least understood. Why is that? Shannon: Because there is a lot of ways to measure it and sometimes just marketing things out there where people will be confused customers. So, as a consumer of displays, whether it’s cell phone displays or TV displays or our big beautiful displays, it’s challenging. How do we compare those? I think, for us in the lab, we’ve got really expensive equipment. We can actually do the measurements. But, as a customer, it’s hard. However, our eyes can pick up contrast ratios really easily. If you see two displays side by side, you can tell that the contrast ratios are different. Matt: If people are going out and measuring the contrast ratio at their site, can the age of their display affect the contrast ratio? Shannon: That’s a good question. Yes. They lose contrast ratio with age, so half of the contrast ratio is how much light the LEDs put out. And so, if you put out less light, you lose contrast ratio. But also, especially outdoors, we have dirt and dust that gets on the face of that and reduces the blackness of the display, and sometimes that can be washed off. Matt: Which is why our services team schedules those display washings for customers, to get the display nice and clean, and keep it with a high contrast ratio. Shannon: It just goes back to one of the constant themes here, there is a lot more that goes into it than what the spec sheet shows. It’s how it was measured, what area, what did they measure it with, the environment. But also, what will it look like in two years? That’s the contrast ratio when you bought it but is it even going to stay that way? Justin: Thanks for joining us, Shannon. We’ve learned that we need to be careful with that spec on the sheet, make sure that you look at displays side by side and make sure you know what lighting conditions you plan for using your display. To listen to this full interview, listen to the podcast episode here.