Why do COB technology LED displays often look redder?

August 8, 2025

If you don’t know what COB is, you can read my previous article, “What is LED COB Display?”

To put it directly, due to the wire bonding process used in most COB LED chips, a portion of the light emitted by the LED chips is blocked. Additionally, because the wavelength of red light is longer than that of blue and green light, more red light will bypass the metal wires and be visible to the human eye.

In people’s minds, light travels in a straight line, but at the microscopic level, photons travel in a curved path.

Light has wave-particle duality.

Yang’s double-slit interference experiment proved that light has wave properties.(reposted from Wikipedia)

The first of Young’s Bakerian lectures was published in the spring of 1802. In 1803, he described his famous interference experiment. Unlike the modern double-slit experiment, Young’s experiment reflects sunlight (using a steering mirror) through a small hole, and splits the thin beam in half using a paper card. He also mentions the possibility of passing light through two slits in his description of the experiment:

Supposing the light of any given colour to consist of undulations of a given breadth, or of a given frequency, it follows that these undulations must be liable to those effects which we have already examined in the case of the waves of water and the pulses of sound. It has been shown that two equal series of waves, proceeding from centres near each other, may be seen to destroy each other’s effects at certain points, and at other points to redouble them; and the beating of two sounds has been explained from a similar interference. We are now to apply the same principles to the alternate union and extinction of colours.

In order that the effects of two portions of light may be thus combined, it is necessary that they be derived from the same origin, and that they arrive at the same point by different paths, in directions not much deviating from each other. This deviation may be produced in one or both of the portions by diffraction, by reflection, by refraction, or by any of these effects combined; but the simplest case appears to be, when a beam of homogeneous light falls on a screen in which there are two very small holes or slits, which may be considered as centres of divergence, from whence the light is diffracted in every direction. In this case, when the two newly formed beams are received on a surface placed so as to intercept them, their light is divided by dark stripes into portions nearly equal, but becoming wider as the surface is more remote from the apertures, so as to subtend very nearly equal angles from the apertures at all distances, and wider also in the same proportion as the apertures are closer to each other. The middle of the two portions is always light, and the bright stripes on each side are at such distances, that the light coming to them from one of the apertures, must have passed through a longer space than that which comes from the other, by an interval which is equal to the breadth of one, two, three, or more of the supposed undulations, while the intervening dark spaces correspond to a difference of half a supposed undulation, of one and a half, of two and a half, or more.

From a comparison of various experiments, it appears that the breadth of the undulations constituting the extreme red light must be supposed to be, in air, about one 36 thousandth of an inch, and those of the extreme violet about one 60 thousandth ; the mean of the whole spectrum, with respect to the intensity of light, being about one 45 thousandth. From these dimensions it follows, calculating upon the known velocity of light, that almost 500 millions of millions of the slowest of such undulations must enter the eye in a single second. The combination of two portions of white or mixed light, when viewed at a great distance, exhibits a few white and black stripes, corresponding to this interval: although, upon closer inspection, the distinct effects of an infinite number of stripes of different breadths appear to be compounded together, so as to produce a beautiful diversity of tints, passing by degrees into each other. The central whiteness is first changed to a yellowish, and then to a tawny colour, succeeded by crimson, and by violet and blue, which together appear, when seen at a distance, as a dark stripe; after this a green light appears, and the dark space beyond it has a crimson hue; the subsequent lights are all more or less green, the dark spaces purple and reddish; and the red light appears so far to predominate in all these effects, that the red or purple stripes occupy nearly the same place in the mixed fringes as if their light were received separately.

Diffraction: refers to the physical phenomenon in which waves deviate from their original straight-line propagation when encountering obstacles.

Simply put, when a wave encounters an obstacle, it will propagate around the edge of the obstacle or scatter in all directions after passing through a small hole, rather than continuing to propagate in a straight line.

	red	green	blue
wavelength(nm)	625~740	492~577	440~475

The wavelength of red light is higher than that of blue and green light, so red light is more likely to bypass obstacles, causing the colors on COB LED displays to appear reddish.