The Grainy Issue
Applications with large particles often appear speckled or grainy. This appearance is mostly noticeable on fine details viewed up close. The grainy appearance is actually caused by several factors and can be minimized in different ways.
Imagine a paper plate covered with a single layer of marbles. In between the marbles, you can see the white of the plate. Now replace the marbles with a single layer of BB's. You can still see about the same amount of white, but the areas are smaller. As the objects get smaller, so do the white areas, known as 'dead space'. As such, this dead space blends in more.
When applied to phosphorescent pigments, smaller particles will lower the grainy appearance. However, larger particles are brighter making smaller particles an option of last resort. You can also minimize dead space issues by using a reflective background surface. A mirrored or white surface will reflect the glow emitting from the rear of the particles. Not only does this increase the amount of light output, it decreases the contrast between the particles and the dead space.
In a lit room, a white background will reflect light onto the rear of the particles. Therefore, you improve charging efficiency. Since you are now fully using the rear of the particle, you can also obtain a brighter glow.
For these three reasons, when the application is thin enough to show dead space, a white or reflective background makes up to a 50% difference in the luminance of the glow. However, as the application gets thicker the background color becomes less important.
Dead space is only responsible for some of the grainy appearance. When looking at a painted surface under a microscope, you will see a series of bumps, hills, and valleys. These bumps increase in size along with the size of the particles. During charging, unless the light source is perfectly perpendicular to the surface, these bumps will cast shadows. Therefore the surface of the hills and bumps facing the light will get charged efficiently. However, the rear of these surfaces and areas in their shadow will receive little or no charging.
Unless you are viewing the glow surface from the exact same angle as the charging light, the surface will look grainy due to the tiny shadows cast during charging. As a visual aid, consider the early black and white pictures of the moon surface where the sun was at a drastically different angle than the camera.
As you can see, there is a trade-off between brightness and the grainy appearance when choosing particle sizes. I would suggest referencing the Which Green article for comparison information. For manufacturing or large projects, I would suggest acquiring multiple samples of our green paints to choose the optimum product.
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