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LEDs - theory

This is the second article devoted to the theoretical foundations of choosing the optimal artificial underwater lighting. The conversation will be about LEDs.

In order to make it easier in the future, you need to start with basic concepts: an LED is an electronic element that, when an electric current passes through it, begins to emit electromagnetic radiation with a wavelength in the visible part of its spectrum or, simply, light. The meaning of the abbreviation becomes clear - LED - light-emitting diode.

Depending on the chemical composition of this electronic component, it emits light with different efficiencies and different wavelengths. Today, the most affordable and efficient LEDs are based on gallium nitride chips. Unfortunately, such an LED emits radiation with a wavelength of 450 - 480 nm, which corresponds to blue.

This problem is solved by using a phosphor. In our case, it is applied directly to the light-emitting element, while other parts of the spectrum appear as a result of re-emission. By combining the composition and thickness of the phosphor layer, manufacturers achieve the required spectral composition of the output light.

We get the following result: at the very bottom there is a heat-dissipating substrate, then the LED case, on it a semiconductor chip, then a layer of phosphor and it is covered with a dome of transparent material that acts as primary optics.

Now let's look at the main characteristics of this device.

In lighting engineering, the main receiver of radiation is the human eye, and, as we remember from the first part, it has different spectral sensitivity. This led to the emergence of a separate description system - a system of light parameters.

One of the main values of this system is the luminous intensity. The unit of luminous intensity is the candela (from Latin - candle). Roughly speaking, 1 candela corresponds to the light sensation that arises when looking at 1 burning candle.

The next important parameter is the luminous flux. The luminous flux determines the amount of light, correlated with the spectral sensitivity curve of the eye, passing through a unit of surface per unit of time - measured in lm.

Since even identical LEDs, due to manufacturing errors and losses during the conversion of light in the phosphor layer, convert electric current with different efficiency, the first possibility of their separation or "bining" by efficiency arises - how many lumens of light the diode produces when consuming 1 watt of energy. Since the efficiency curves of LEDs, depending on their heating and transmitted current, are not linear, some fixed values of these parameters are chosen to ensure the correct comparison.

Due to the fact that radiation of the same power, but having a different wavelength, is perceived differently by the eye, the luminous flux can be estimated in lumens only in those cases when the spectral composition of the light does not play a significant role, and only the brightness of the illumination perceived by the eye is important. This option does not suit anyone and therefore two additional parameters were introduced - this is CCT - correlated color temperature and CRI - color rendering index.

The simplest description of color temperature is comparing this parameter to the light emitted by a heated metal. Different heating temperatures give different spectral composition of radiation - higher temperature - whiter light. Only in our case, it was not the metal that was heated, but an object called an absolutely black body. Let's not talk about it - it will not affect understanding in any way.

The heating scale was divided into intervals and the spectral characteristics were taken for each of them. Now it has become possible, when describing any light source, to compare its spectral characteristics with those obtained upon heating. And finding the most similar to say - this light source has a CCT for example 5000K.

But since there will be no exact match, it becomes necessary to standardize these deviations. An example of this is the American National Standards Institute (ANSI) color standard C78.377A. This standard divides the temperature range into 8 parts and defines the permissible ranges of its deviation.

Temperature deviation ranges are essentially defining ranges of color hue deviation. And they can be noticeable, so that each specific range of deviations is also divided into sectors.

In practice, it looks like this: take the so-called chromaticity diagram (the essence of this diagram is that any existing color can be described using two coordinates x and y) and superimpose on this diagram the glow curve of an absolutely black body depending on temperature. Since, according to this standard, there is a certain range of possible deviations in temperature, we get not a line, but a certain surface. This surface is divided into small sectors, color "bins". After that, a group of "bins" belonging to the same temperature range are combined into a group or "kit". "Kit" can combine not the entire temperature range, but only part of it. The description of a "kit" or a specific "bin" is the x and y coordinates superimposed on the chromaticity diagram. This chromaticity standard is not the only one, but they have the same meaning - they all divide the chromaticity diagram into sectors with coordinates.

Now the color rendering index CRI - it determines how good the light from the source is for the correct perception of the color of the object. To determine this parameter, a set of 8 standard samples with different colors is illuminated with the tested source and the reference one with the same color temperature. If there is no difference, the CRI is 100. The more samples have changed the color, the less it is. CRI values above 80 are believed to be necessary for good color rendering.

All of the above is necessary for us only for one thing - we must clearly understand what the letters and numbers mean in the designation of a particular diode. Therefore, we move from theory to practice.

Today, the American company CREE is the leader in the production of high-performance full-spectrum or "white" LEDs, and with a very high probability we will encounter them. Therefore, it makes sense to apply our knowledge when choosing an LED from this particular manufacturer.

So, we go to the company's website, and try to figure it out, for example, with the marking of the XPG3 diode. We go to the product card, look at its characteristics and if we are interested in them, we proceed to choosing a specific option. To do this, open its datasheet and look. In most cases, the information displayed in the table, which starts in this case from the third page, will be enough for us. As you can see, all the parameters of this table are well known to us. In the table, we can select the temperature of the diode glow, its efficiency and color rendering index.

If this is not enough, then a full description of the nomenclature of order codes can be found on page 27 in the section “Formats of codes and order codes”, and to define color sets or “kits”, see the table of color cells on page 26.

I will tell you how important or not the color rendering index and chromaticity set in underwater lighting, as well as the selection of the optimal characteristics of LEDs, in the next part

See you!

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