LED and photodiode

LED

LED physics: as shown in Fig. 1, current flows from the p-side (anode) to the n-side (cathode) when the diode is forward biased. When an electron meets a hole (electron-hole combination), it falls into a lower energy level, and releases energy in the form of a photon.


(Image is from: http://en.wikipedia.org/wiki/Light-emitting_diode)

The wavelength of the light emitted, and therefore its color, depends on the band gap energy of the materials forming the p-n junction.

Photodiodes

Photodiodes are one type of photo detectors (other types includes thermal detectors, photoresistors, photomultipliers, etc.).

Same as LED, a photodiode is a semiconductor p–n junction device. But opposite to LED, in a photodiode light is absorbed in a depletion region and generates a photocurrent. There are two operation modes for a photodiode:

Photovoltaic mode: operated in zero bias. The illuminated photodiode generates a voltage which can be measured. This is photovoltaic effect, which is the basis for solar cells—in fact, a solar cell is just an array of large area photodiodes.

Photoconductive mode: operated in reverse bias (opposite to LED). The resulting photocurrent can be measured. Detectors operated in this mode have high linearity and dynamic range.

Material	Spectrum range
Silicon (Si)	400-1000 nm
Germanium (Ge)	900-1600 nm
Indium gallium arsenide phosphide (InGaAsP)	1000-1350 nm
Indium gallium arsenide (InGaAs)	900-1700 nm

References:
[1] http://en.wikipedia.org/wiki/Light-emitting_diode
[2] http://www.rp-photonics.com/photodiodes.html

Luminescence - photoluminescence - phosphorescence

Luminescence refers to light emission caused by electron transition from excited state to ground state; not by a rise in temperature (light emission from pure heat source is incandescence).

By different excitation(pumping) mechanism, luminescence has various types, for example:

• Photoluminescence: material is pumped by absorption of photons (e.g., fluorescent lamp).


• Cathodoluminescence: pumped by electron beam (e.g., CRT monitor).


• Electroluminescence: pumped by electric current or field (e.g., LED).

Photoluminescence:
From the transition speed, photoluminescence is divided into two types:

• Fluorescence: an almost instantaneous effect; the emission ends within ~10^-8 second after excitation.


• Phosphorescence: this term describes the persistent luminescence (afterglow) of phosphors (the "glow-in-the-dark" phenomenon). The upper state is either metastable with long lifetime or is transition forbidden to the ground state.

Phosphorescence:
Materials exihibiting phosphorescence are known as phosphors. Phosphors are usually some microcystalline materials. In these crystals, there are impurity ions called activators, which replaces some host ions in the crystal lattice. These activators form luminescing centers and are where the excitation-emission process occurs.

References
[1] John Wilson and John Hawkes, Optoelectronics, an introduction, 3rd ed., Prentice Hall Eruope, 1998. Chapter 4.
[2] Luminescence article at www.britannica.com
[3] Other sites that explain the phosphorescence physics:
http://math.ucr.edu/home/baez/spin/node17.html
http://en.wikipedia.org/wiki/Phosphorescence

Photometric meters

1. Goniophotometer [1]
The Greek word "gonio" means angle. A goniophotometer measures spatial distribution (xyz and angular) of a radiation source. It has a built-in photopic [2] filter so all measured radiometric quantities automatically convert to photometric ones after calibration. A goniophotometer is able to measure:

• Luminous power (flux) in lm.


• Luminous intensity in cd = lm/Sr.


• Illuminance in lux = lm/m².


• Luminance in cd/m² = lm/m²-Sr.


• Color coordinates and correlated color temperature.


• Sample's retro-reflection as function of angle in cd/lx.

2. Colorimeter [3]
There are also filters in a colorimeter to mimic the human cone response. A colorimeter produces numerical results in one of the CIE color models.

References and notes:
[1] X-Rite GmbH - Optronik catalog.
[2] The word "photopic" refers to vision by cone receptors in the human eye (while "scotopic" refers to rod receptors).
[3] Bruce Fraser, Chris Murphy, and Fred Bunting, Real World Color Management, 2nd ed. Peachpit Press 2005. Page 43.

Radiometry and photometry

Radiometry means the measurement of radiation. Photometry is a modified radiometry when wavelength is weighted by how sensitive the human eye responses.

Both radiometry and photometry have their own tricky/bizarre names for basic quantities:

1. Radiometry quantities:

Name	Other name	Units	Symbol
Energy		joule (J)	Q
Flux	Radiant power	watt (W)	Φ
Irradiance	Flux Density	W/m2
Radiant exitance		W/m2	M
Radiant incidance		W/m2	E
Radiant intensity		W/Sr	I
Radiance		W/(m2Sr)	L

Descriptions:
Flux (radiant power): time rate of change of energy.
Irradiance: areal density of power. I noticed more and more scientific articles have used this term to replace "intensity", which is good.
Radiant intensity: power per unit solid angle.
Radiance: power per unit projected area per unit solid angle. (This is equivalent to brightness often used by laser people).

2. Photometry quantities:
Same as radiometric quantities but units have different names:

Radiometric name	Photometric name	Radiometric units	Photometric units
Energy		joule (J)
Radiant power (flux)	Luminous power (flux)	watt (W)	lumen (lm)
Irradiance	Illuminance	W/m2	lm/m2 = lux (lx)
Radiant intensity	Luminous intensity	W/Sr	lm/sr = candela (cd)
Radiance	Luminance	W/(m2Sr)	lm/(m2Sr) = cd/m2 = nit

Descriptions:
Candela: (unit of luminous intensity) one of the seven base units of the SI system. If a monochromatic 555nm source emits 1 W per steradian at a given direction, then at that direction the luminous intensity is 683 candelas (or 683 lm/sr). 555 nm is the wavelength that human eye has the max spectral responsivity.
Lumen: (unit of luminous power) For an isotropic source having 1 candela luminous intensity, the total luminous power emitted is 4π. If a source is not isotropic, one needs to measure the luminous intensity in many directions using a goniophotometer, and then numerically integrate over the entire sphere.
Lux: (unit of illuminance) = lm/m². Most light meter measures this quantity.

3. Conversion between Watt and Lumen
The simplest thing to remember is:
1 Watt = 683 Lumens @555 nm.
At other wavelengths, this value is smaller and we need to multiply the eye's spectral response curve V(λ).

References:
[1] There is an excellent article on this subject by late Professor James M. Palmer:
http://www.optics.arizona.edu/Palmer/rpfaq/rpfaq.pdf
[2] The book Introduction to Radiometry (SPIE Optical Engineering Press 1998) by William L. Wolfe is a full-range detailed reference. I found some slight differences between the two references on quantity naming. This means that this subject is still somehow not coordinated.

Opponent colors

There exist three opponent color pairs:
Light-Dark;
Red-Green;
Yellow-Blue.

We cannot have something that is both light and dark, or red and green, or yellow and blue (but we can have reddish-yellow (orange) and blueish-red (purple), etc.). The two colors in each pair are totally opposite or exclusive. This fact enables us to establish LAB, a 3-D color space, in which these three pairs are the three axes.

Structure below shows the relationship. The diagonal colors are opponent pairs:

     (O)
    R - Y
(M) |   |
    B - G
     (C)

       (橙)
      红--黄
(品红) |   |
      蓝--绿
       (青)

It seems that there should be four basic or root colors (RYBG), however Y can be produced by mixing R and G (this makes sense as Y resides between R and G in spectrum, so the overall response from eye cone receptors is yellow) so Y is not a primary color.

Anyway, opponent colors are very mysterious to me.

Opponent colors appear in our afterimage or ghost image, which refers to an image continuing to appear in one's vision after the exposure to the original image has ceased [2].

Are opponent color pairs the same as the inverse colors in the negative film? I don't think so. The three primary colors in negative film should be CMY.


References:
1. Bruce Fraser, Chris Murphy, and Fred Bunting, Real World Color Management, 2nd ed. Peachpit Press 2005
2. www.absoluteastronomy.com/topics/Afterimage

Brightness, hue, and saturation

These are the three attributes of color.

Brightness is the achromatic component, i.e., light power or intensity that detected by our eye.

Hue and saturation are the chromatic components. Simply defining,
Hue = wavelength;
Saturation = spectral purity.

Hue:
The wavelength that appears most prevalent in a color sample determines its hue. The set of basic hues, for example, 赤橙黄绿青蓝紫, is very subjective and differ from culture to culture.

Saturation:
"Spectral purity" is enough to define saturation. Lasers produce the most saturated colors whereas white-gray-black are the least saturated colors.

References:
1. Bruce Fraser, Chris Murphy, and Fred Bunting, Real World Color Management, 2nd ed. Peachpit Press 2005

Eye and color event

1. Eye structure:

(Image is from: http://www.schools.net.au/edu/lesson_ideas/optics/optics_wksht2_p1.html)

Of all these names I should know the most important ones as an optical scientist:

Cornea: focusing light to form an image (together with lens; but cornea plays the major role on focusing [1, page 16]).

Iris: aperture.

Lens: besides focusing adjustment, it also acts as a UV filter to protect the retina.

Retina: see next section.


2. Retina

There are two types of nerve cells, or receptors, in the retina:

Rods: provide vision at low light and has a peak absorption at 499nm. It is color blind.

Cones: provide color info. The three types of cones, RGB, have peak responses at 420nm, 530nm and 565nm respectively.

(Image is from: Eysenck, Cognitive Psychology: A Student's Handbook)

Therefore one can stimulate almost any colors by using just three well-chosen primary colors.

Additive primary colors - RGB:
starting from black (no wavelengths), adding R,G,B one by one, we obtain white light (all wavelengths).

Subtractive primary colors - CMYK:
starting from white,
subtracting cyan, we get red (cyan ink is "red-subtractor" or "long-wavelength subtractor");
subtracting magenta, we get green (magenta ink is "green-subtractor" or "medium-wavelength subtractor");
subtracting yellow, we get blue (yellow ink is "blue-subtractor" or "short-wavelength subtractor").

Opponent color pairs:
This is very mysteries to me. The opponent color pairs are:
Light-Dark;
Red-Green;
Yellow-Blue.

3. Color event

Strictly speaking, color is an event. It is a product of three things: light, object, and observer.

4. Metamerism

Metamerism is a phenomenon that two incident lights with different spectra produce the same color sensation by human eye. For example, a blend of R and G produces Y but this is different from a pure Y produced by a yellow laser, although both appear the same color, yellow, to our eye. Another example is that two clothes having the same color in store may become different colors viewed under sunlight or at home. This is because of the limitation of our eye as a spectrum analyzer. Our eye divides the incident light into only 3 components by the R,G,B receptors, whereas an optical spectrum analyzer is able to divide the incident light into many pieces. In other words, our eye's resolution bandwidth is very crude and this causes metamerism.

Metamerism is good! Why? Because if without metamerism, our printers would need many inks in all different colors, instead of just four (CMYK).

References:
1. Bruce Fraser, Chris Murphy, and Fred Bunting, Real World Color Management, 2nd ed. Peachpit Press 2005