eInk Displays and Spectrophotometry
I’ve been researching ways to judge eInk display brightness, which is a very important stat for an eInk device. The brighter the display’s background is, the better the contrast. The better the contrast the easier it is to see. Things that are easier to see reduce eye strain and just look better. Before now no widely available chart told you what the metrics were from standardized tools. Just observations, some random formulas, and measurements that weren’t repeatable under normal circumstances. Standardized is the keyword here. Once you have standardized readings taken from certified instruments done in a repeatable manner, it makes talking about these topics easier.
I’ll be using a few instruments in my reviews and articles, for taking readings on devices and their components. The first device I’ll be talking about is called a Spectrophotometer. A Spectrophotometer allows me to test sample whiteness and brightness, by measuring their light absorption properties.
“In simple terms, a spectrophotometer is a tool that helps scientists and researchers study the properties of light and how it interacts with different materials. By measuring the amount of light absorbed by a sample at different wavelengths, a spectrophotometer can provide valuable information about the composition and properties of the sample.
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I plan on using this device for “Whiteness” and “Brightness” testing. Afterward, I’ll be doing some “Ghosting” tests with it. A database will come soon after, where anyone can go and look up display data on a device they’re researching.
DS62 Spectrophotometer
With that said, I’d like to go over my methodology for measuring eInk screen brightness and whiteness. Firstly, the device I use is called the DS62 Spectrophotometer. It uses the CIE (International Commission on Illumination) standard to measure brightness/whiteness, among other things. It does this by passing light through a sample and measuring its absorption rate. I measure each sample using the CIE (Whiteness) setting, which measures the “whiteness” of objects. I used this setting to measure how close to white each sample is.
I also measure each sample using the CIE “Lab” setting, which measures the light reflected off a surface. I used this because eInk devices use ambient light for illumination. Ambient light works by reflecting the surrounding light off a surface, to your eyes. Since that’s exactly what CIE “Lab” measures, I chose that to measure the brightness. However, Brightness is only the “L” component in the “Lab” test. The other two components, “a” and “b”, are indicators of color shifting. Negative “a” values indicate a green color shift, while positive “a” values indicate a red color shift. Negative “b” values indicate a blue color shift, while positive “b” values indicate a yellow color shift. This can give us some sense of what hue the display will have.
The Parameters:
Formula = dE*ab: For detecting general color differences in samples
Angle = 10º Angle: Standard observing angle. Better suited for reflective surfaces like eInk screens that reflect light.
Light = D65: Standard daylight exposure. A common choice for exposure types.
Mode = SCI (Specular Component Included): This means that the gloss or shine of the sample is included in the measurement.
All sample measurements are an average of 5 individual measurements.
All other parameters and settings were left in their default states.
The Readings:
Whiteness: Whiteness refers to how close the sample is to white. On a scale of 0-100, where 0 is black and 100 is perfect white.
Brightness (L): A measure of how much light is reflected off of a sample. This is a judge of the surface’s brightness. On a scale of 0-100%, where 0 indicates no reflected light, and 100 indicates that all outbound light was reflected.
Color Shift (a): Negative values indicate a green color shift, while positive values indicate a red color shift.
Color Shift (b): Negative values indicate a blue color shift, while positive values indicate a “yellow” color shift.
This was an overview of my method for gathering data on eInk displays. It’s just a starting point, and may not be perfect but with time, and the help of the eInk community, I’m sure it will get better. I plan to record the calibration process as a YouTube video so get subscribed if you’re interested in that kind of content. The goal is for it to get this to a place where it can be an additional resource for eInk information. Let me know your thoughts below or in the feedback section.
