Artifical Light and Light Measurement

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Lecture 17: Lights and Light Measurement

Light = EM-energy, radiated.

UV - ultraviolet: 200-400 nm, not visible to human eye, but absorbed in tissue. UV-A: 320 – 400 nm, UV-B: 290-320 nm; germicidal: 256 nm (Mercury emission line)

IR - infrared: above 700 nm (far-red typ. 700-800 nm), “heat”

Visible light for humans: 400 nm (end of blue) to 700 nm (end of red)

Sun light equivalent to 6000K radiation from sun, with peak at 555 nm (green)

Peak Wavelength Wien’s law: proportional to temperature;
sun = 6000 K = 555 nm / green.
light bulb » 1000 K > 1,100 nm / peak in IR (more hot than light.

Human Vision - Color Vision

The color-responsive chemicals in the cones are called cone pigments and are very similar to the chemicals in the rods. The retinal portion of the chemical is the same, however the scotopsin is replaced with photopsins. Therefore, the color-responsive pigments are made of retinal and photopsins. There are three kinds of color-sensitive pigments:

· Red-sensitive pigment

· Green-sensitive pigment

· Blue-sensitive pigment

Each cone cell has one of these pigments so that it is sensitive to that color. The human eye can sense almost any gradation of color when red, green and blue are mixed.

In the diagram above, the wavelengths of the three types of cones (red, green and blue) are shown. The peak absorbancy of blue-sensitive pigment is 445 nanometers, for green-sensitive pigment it is 535 nanometers, and for red-sensitive pigment it is 570 nanometers.

Perception is the differentiation of figures from their backgrounds. In the visual world, color differences provide the means of distinguishing between fore ground and background. There are two types of color, namely achromatic color and chromatic color. The color that stands out just because of its greyness (will be called as lightness) is called a achromatic color. There is no component of hue in it. Chromatic colors are those that have a hue component to them.

Before proceeding to the concept of achromatic and chromatic color, it is very important to first discuss the monochromatic color. Monochromatic color ideally refers to light of a single wavelength. This is very difficult to achieve physically. It is known that the color sensations reported by an observer with normal color vision vary as a function of wavelength of the light stimulus. This holds true for any wavelength in the range of 400-650nm. The sensations reported by the observers exposed to the various wavelengths is known as hues. In 1976, Murch and Ball performed an experiment. Subjects were asked to identify the colors of light made of very narrow wavelength band (10nm), covering the whole visual spectrum. The observers were asked to characterize each stimulus with four numbers corresponding to the amount of blue, green, yellow and red perceived to be present in that particular target. The results of the study showed that the human perception is insensitive to wavelengths of less than 400nm and above 650nm. So 400-650nm is called the visual spectrum of light. In 450-480nm, the predominant sensation was that of blue. Green has a fairly broad band from 500-550nm. Yellow was concentrated in a narrow band of 570-590nm. Wavelengths above 610nm were characterized as red. Another important observation was, that most of the colors were characterized with more than two categories. For example a 500nm stimulus was given an average rating of green, blue, yellow and red. The best or purest colors - defined as the maximum value estimated for one color category and minimum value for the other three categories - indicated pure blue at about 470nm, pure green at about 505nm and pure red at about 575nm. This is illustrated in Figure 1.

Figure 1: The color spectrum of light for different wavelength

Additive and Subtractive Color Wheel

ADDITION - Primary Colors of LIGHT: red, green, blue.

Example: Mixing of the three primary colors of light = addition (primary color overlaps): yellow = red + green, red+blue+green = white. TV-monitor, screen: use additive color mixing.

SUBTRACTION: Primary Colors for mixing pigments or dyes used in coloring, photography, and printing are: CYAN, MAGENTA, YELLOW. The magenta, cyan, and yellow of mixing pigments and dyes subtracts colors to create new hues. This is referred to as subtractive color mixing. The dyes of inks absorb certain colors. Any color that is not absorbed (subtracted) is the hue that we see. These dyes act as filters that subtract one or more colors. By varying the proportion of the colors in a mixture, a full range of colors can be produced.

CIE Color:

Introduction: In 1931, the Commission Internationale de l'Eclairage (CIE) developed a device-independent color model based on human perception. The CIE XYZ model, as it is known, defines three primaries called X, Y and Z that can be combined to match any color humans see. This relates to the tristimulus theory of color perception, that states that the human retina has 3 kinds of cones with peak sensitivities to 580 nm ("red"), 545 nm ("green") and 440 nm ("blue"). These primaries are combined in an additive manner, an advantage over other color models (such as RGB) where negative weights may be needed to specify some colors.

The Y primary was defined to match the luminous-efficiency function of the human eye. X and Z were obtained based on experiments involving human observers. The chromaticity values are defined as:

x = X / (X+Y+Z)

y = Y / (X+Y+Z)

z = Z / (X+Y+Z)

Knowing x and y, z can be found as z = 1-x-y.

The CIE Chromaticity diagram (shown below) is a plot of X vs. Y for all visible colors. Each point on the edge denotes a pure color of a specific wavelength. White is at the center where all colors combine equally (x = y = z = 1/3).

The CIE definition of the standard observer is based on three specific wavelengths of light in the RGB regions respectively (435.8, 546.1 and 600nm, see the figure below).

http://hyperphysics.phy-astr.gsu.edu/hbase/vision/cie.html

COMMISSION INTERNATIONALE DE L'ECLAIRAGE / INTERNATIONAL COMMISSION ON ILLUMINATION / INTERNATIONALE BELEUCHTUNGSKOMMISSION http://www.cie.co.at/cie/

Intensity

W/m² Physical unit; energy per time and per area (flux); independent of wavelength, total amount of energy integrated across spectrum.

PAR / Photon flux photobiology – photosynthesis: number of photons per area per time (photon flux) in photosynthetic active range: 400-700 nm. Not defined below 400 / above 700 nm). Measured photons independent of their individual energy content (E = h * n), because for photosynthesis, a blue, more energetic photon (lower wavelength = higher frequency) has similar rates of photosynthesis than a red, less energetic photon. Thus, only the number of photon counts, not their energy content (in broadband light – ‘white light’). Be careful when using this assumption with narrowband (‘monochromatic’) light.

Lux only usefull / defined for visibility of human eye from 400 to 700 nm, peak 555 nm. Not defined outside this wavelength. For human eye, the same radiant energy flux (W/m²) is perceived brighets for green light (555 nm), and almost invisible / less bright in the blue (400-450) or red end of spectrum (650-700 nm).

Unit Conversion depends on spectral composition. If the spectrum is known (spectro-radiometer, W/m2 per nm), than one can convert, for each wavelength band, from W/m² to lux or PAR.

Some rules of thumb can be applied for known, broadband light sources:
Full sunlight = 100,000 lux = 450 W/m² = 2,000 mol/m²/s.

Spectral Composition

Color different spectral composition (intensity at each wavelength) is perceived as different color.

Some light sources produce narrow bandwith light, such as spectral emission lines of mercury 256 nm, Sodium,.. or LEDs (typ. 20-40 nm bandwidth at 50% intensity.

Other light sources have broad (white) emission – filament light bulbs, fluorescent lights, high pressure sodium lamps, arc lamps.

Typical Light Sources

Light bulb (filament) glowing filament; temperature-dependent peak wavelength, very broad, with peak in IR, very little blue / UV content.

Vacuum vs. inert gas vacuum bulb – prevent filament from burning up (oxygen). Inert gas prevents oxidation (burn-up of filament), and aids in convective re-deposition of evaporated filament wire material, potentially extending the life time. Dark deposits in filament bulbs is typically condensed filament material.

Fluorescent lamp Internally, a mercury spectral emission (accelerated electronic collide with Mercury vapor, releasing 256 nm UV light). Mercury lamps without coatings are UV-lamps (germicidal lamp – 256 nm, UV-black light 350 nm). Special glass is necessary to transmit UV-light though the walls (many materials absorb UV light). When coated with phosphors on the inside (mixtures of three basic phosphors are used – blue, red, xx - to make different types of ‘white’ light), the 256 nm UV-radiation is converted to visible light. For these visible light lamps, a glass that does not transmit UV light is chosen.

LED Light emitting diode. Color is very narrow bandwidth and depends on diode material. Longer wavelength LEDs are more efficient / higher intensities (telecom IR LEDs, red LEDs).

Efficiencies

Electric to light conversion of electric energy (P = V * I) to radiant energy. Only use visible light portion, UV and IR (not visible) is regarded as ‘waste’.
Low Pressure Sodium (30%), High Pressure Sodium (30%), Fluorescent (20%), LED (5-10%), Halogen (<5%), Incandescent (1-5%).

Power conversion different light sources require different power conditioning, such as DC or AC voltage at different levels (power conversion), or current limiting features (‘ballast’ for fluorescent lamps, current limit for LEDs). This ‘conversion’ equipment takes up space, mass, and power (AC inverter for fluorescent lamps: 80% efficiency; voltage converters: 70-90% efficiency) – therefore, 10-30% of initial electric energy is lost before you get to lamp.

Distribution efficiency Light from light source may require directional changes to reach target area – reflectors, mirrors, diffuser, light pipes,…A very good mirror may be 90-95% efficient, a good scientific white diffuser may reach 90-92% reflectivity.

Other Considerations

Available Intensity Light sources are only available in descrete size

Dimming Intensity control can be accomplished by changing / controlling the power available to the light sources. Some lamps may have limited controllability (i.e., not linear from zero to full, but may only be dimmable from 50 to 110%. 100% = recommended max. power at rated life time. Overdriving typically reduces life time.
Dimming circuits typically reduce efficiency, and may involve controlling the current, the voltage or the frequency (pulse width / duty cycle) of the electric power unit.

Life time LED 100,000 hr, fluorescent lamps (10,000-20,000 hrs), incandescent lamps (40 hrs to 2,000 hrs. typ). Consider mass and logistics of replacement items.

Reliability Depending on manufacturing process or environmental compatibility (vibration, shock, over- / under-temperature), light sources may be subject to failure (broken glass, filament broken / burned out, manufacturing tolerances)

Temperature dependency LEDs fail as low as 50C (brighter at lower temperature, while fluorescent lamps increase in intensity up to 80C, then drop off quickly at 100C. Fluorescent lamps without pre-heat circuit may not work at low tempetures

Policies Toxic contents (mercury vapor), high pressure (high pressure sodium lamps), high temperature (filament, high power sodium lamps), may favor more ‘inert’ / ‘conventional’ / ‘safer’ light sources - LED