Home / Halogen Point Heater / Basic knowledge of Halogen heater / 5.Physics of light heating / 5-1.About light reflection, transmission, absorption, and radiation

5-1.About light reflection, transmission, absorption, and radiation

Light Reflection, Transmission, Absorption

Light incident on the object shown in the figure above is partially reflected, some is absorbed, some is transmitted.

Ratio of the reflected light if the A%, reflectance of the object will be A%. Absorption rate of B%, transmittance is C% as well.

A+B+C will certainly be 100%.

This is the law of conservation of energy.
Even if energy changes a form , the quantity of energy also does not increasing and does not decrease.

( Incident light energy) = ( Reflected energy) +( Absorbed energy) + (Transmitted energy)

The emissivity is the fixed quantity of radiation phenomena.
When it becomes to a temperature at which there is an object, energy is emitted in the form of (mainly microwave-infrared to visible light region) light from there, emissivity is a number of this radiation degree.

The black body is an idealized physical body that absorbs all incident electromagnetic radiation, regardless of frequency or angle of incidence.

It is easy to radiation as same as easy to absorption.
If the absorption rate is zero, the object can not heat dissipation by radiation.
The important point is always “absorption rate = emissivity rate”.
This is the Kirchhoff’s law (radiant energy).

Characteristics and varies with temperature even in the same glass radiation of the glass (= absorption) characteristics.
Silica glass is transparent to the visible light region at room temperature, but is close to being opaque becomes (1200 ℃) softening temperature, I will be as glowing yellow.
Quartz glass does not light remains zero even visible light emissivity because it is still clear at 1200 ℃.
However, because it is opaque in the infrared region, and infrared radiation.
Absorptivity (= emissivity) is going to become higher in the visible light range from around 1700 ℃ also quartz glass, I will begin to light in bright white light.
I estimated temperature because I feel washed out than (6000K) sunlight But 2000 ℃, the emissivity of the red and nears low.

Relationship of emissivity = absorptivity index of the material and electromagnetic wavelength

The above figure shows the trend with respect to the wavelength of emissivity (= absorptivity) of metals and nonmetals.
This is a general trend, the value will vary greatly depending on the individual substances.

Infrared heating of metal

Absorption rate is reduced as the wavelength becomes longer metal, and is not suitable for heating of the far-infrared heater.
And because you are suitable for heating of the near-infrared wavelength, is the best (about 1μm peak wavelength) halogen lamp heater.
Yet there is also a metal close to impossible heating of a metal material.
For example, the reflectance is higher in the visible to near infrared region, such as aluminum or copper, it is difficult to heat conduction heating also since it is a bonus.
However, these can also be heated by a surface state.
(For example, a fine irregularities, oxidative discoloration, hairline and satin.)

Infrared heating of non-metallic

Absorptivity-emissivity increases in the far infrared region shown in the figure above in general, far-infrared heater is more appropriate non-metallic.
However, it is not possible therefore to high temperatures in the far-infrared heater, halogen heater output per unit area often is suitable if it want to high temperature rapidly.

Virtual object “black body” is defined as 100% in the whole wavelength band, powdered carbon closer to it’s (95-98%).

Emissivity = absorption rate is 95% or higher (about 10μm) far-infrared wavelength region emitting at body temperature human body.
There tends to be high organic matter absorption of this wavelength region.
It is believed that in order to survive the Ice Age, it’s a result of (= organic matter) have evolved organisms to seek blessings of the sun.

Energy is emitted in the form of radiation as represented by the formula Stefan Boltzmann and Planck reaches a certain temperature an object.
However, heat energy is to move I will occur for the first time if there is a temperature difference.
For example, the human body is the radiator very good, but you can heat dissipation to the surroundings, but only if the ambient temperature is lower than body temperature.
I feel to be “cool” heat dissipation along the emissivity for the first time when there is a temperature difference between the ambient (heat transfer) occurs.
(In this case, better heat dissipation by wind and convection is higher percentage than the heat dissipation by radiation, but does not go into it here)

It’s impossible heat dissipation when body temperature and the ambient temperature is the same, because there is no heat transfer at zero deduction from received from around the energy of only the same energy that was radiated to the surroundings.

If you think this way, why not warm to such should have a temperature radiation corresponding to the temperature of any object from the cold? I think the question of also eliminated.
From the object having a temperature that is certainly, you receive radiant energy corresponding to, and the thermal radiation corresponding to the temperature of the object itself to be at the same time from their.
I feel cold and my energy has worn better towards you pass temperature is higher than your opponent because there are many.
It is that those who receive the temperature of the opponent is higher than their own is increased, and that, feel warm thermal energy is increased.
In this case, the heat transfer is faster emissivity and absorption rate of the other side is high.

So easy to understand and likened to a human body, it was described as ” I “.
The same is true even if replaced with anything.
I feel warm that will temperature is increased, so “I feel cool feel” that, when the temperature is lowered.

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