You’re at your favorite sports bar. Its 1:00PM and your game just kicked off. It’s a nice day so you’re out on the patio. Unfortunately, the sun is shining bright, and you can’t see anything on the TV hanging on the wall. The bar owner has no idea why he has a problem; he bought a high-end outdoor TV putting out 2,000cd/m2 of brightness!
So why can’t you watch the game?
Optical Bonding Enhances Outdoor Viewability
The answer is that this setup doesn’t include a key component for outdoor viewability – known in the LCD industry as Optical Bonding. An optical bonding gel is a transparent epoxy or silicone glue that goes between the front of the LCD and the protective glass in front of it. In order to understand why we want to add extra glue in between the LCD glass and front protective glass, we need to take a short dive into the physics of reflection.
When ambient light (like from a light bulb, or the sun) hits a transparent object, some of that light is reflected. Specifically, when ambient hits a piece of protective glass in front of an LCD, around 5% of that light is reflected. This happens because the medium through which light is traveling changes (from air to glass), and the light reacts differently to the new material’s density (index of refraction). 95% of the light passes through the protective glass and hits air again (the air between the protective glass and the LCD glass). You get another 5% reflection. When that light then hits the front of the LCD glass – you guessed it – another 5% reflection.
OK, so we lose 15%, what’s the big deal? I have a 2,000cd/m2 display and 1,700cd/ m2 is still more than sufficient. The problem is that the reflections aren’t 15% of the display’s brightness being reflected. It’s 15% of the Sun’s brightness (or whatever is creating the ambient lighting). At noon in direct sunlight, that is around 32,000 cd/m2… so 15% of that is around 4,800 cd/ m2. All of a sudden that 2,000cd/ m2 display seems a bit outclassed, and this is why the image appears washed-out.
If The Standard LCD Display Brightness is Increased it Becomes Expensive and Will Eventually Burn Out
So what can we do? One path would be to keep increasing our brightness. What would it take to see through 4,800 cd/ m2? 15,000? 20,00? Unfortunately, that is going to be very expensive in both components and in power consumption. It is also is going to get so hot that the display will burn itself out and be worthless. Increasing brightness indefinitely to fight the sun is simply the wrong approach to sunlight visibility.
So what should we do? First we deal with the very first reflection – the front surface. An anti-glare etching causes the reflected light to spread into lots of different directions. If the reflection goes off in random directions and (mostly) doesn’t get back to your eye, it isn’t a problem – let’s say the front reflection becomes 1% instead of 5%. That still leaves another 10% to deal with! Stopping there still leaves a huge problem. That’s where optical bonding comes in.
A good optical bonding material has an index of refraction very close to that of glass. By completely filling the volume between the front protective glass and the LCD glass, we trick the light into not seeing a reflective surface at all. That 10% reflection is gone. Assuming we did everything right, we only need to fight against 320cd/m2 of sunlight at noon instead of 4,800 cd/m2.
So how bright should our monitor displays be? That is a matter of contrast. Contrast for an outdoor display is a ratio of brightness to reflectance. For the best possible experience (looks the same as a TV in a dark room) we would target a ratio of LCD brightness to reflection brightness of around 15:1. Going higher won’t net any extra performance boost. In a pinch, we can generally get by with a ratio of around 6:1. Figuring that a 350 cd/m2 environment is the worst case scenario, and some measure of shade (awning, clouds, simply not being noon) improves that figure greatly, our 2,000 cd/m2 optically bonded display is more than sufficient.