E5 Understanding The Theory

E5 UNDERSTANDING THE THEORY

What is Photopic and Scotopic Vision?

The retina, a light sensitive membrane at the back of the eye, contains millions of very tiny light receptors that convert light into electrified signals sent to the vision centers of the brain. The two major categories of light receptors (photoreceptors) are called cones and rods because of their shapes. The very central part of the retina, the fovea, contains only cones. The rest of the retina contains both rods and cones, with the number of rods dominating the cones by about ten to one.

Up until now, it’s been widely accepted that cones handle day vision and rods are designed for night vision. Consequently, lighting manufacturers have utilized light meters to measure a lamp’s lumen output that are calibrated by examining the eye’s sensitivity to only cone activated vision in the very central part of the retina (photopic), completely ignoring the effect of rod activated vision (scotopic).

But, according to a study by Dr. Sam Berman and Dr. Don Jewett, the roles of rods and cones are not that exclusive - they actually share responsibility depending on lighting conditions. Dr.’s Berman and Jewett’s experiments, sponsored by the U.S. Department of Energy, have shown that rods (scotopic) do indeed play a role in typical workplace lighting conditions. Thus, human perception of lighting conditions is not consistent with the devices we generally use to measure light output.

This and other studies lead us to the conclusion that both photopic and scotopic responses to lighting need to be evaluated when measuring light effectiveness. Ideally, this would require light meters with a calibration for conventional (photopic) illuminance as well as an addition calibration for scotopic illuminance.

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a. Definition of Light
Definition
• Light is that which makes things visible.
• Light is defined as electromagnetic radiation or energy transmitted through space or a material medium in the form of electromagnetic waves (definition in physics).
• Light is defined as visually evaluated radiant energy – light is that part of the electromagnetic spectrum visible by the human eye (illuminating engineering definition).

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Electromagnetic Spectrum
• The electromagnetic spectrum is shown in the figure below.
• The visible portion of the spectrum covers a narrow band of wavelength from approximately 380 nm to 770 nm
(1 nm = 10-9m). Wavelengths shorter or longer than these do not stimulate the receptors in the human eye.

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b. Visual Effect of Light
• Light is defined as visually evaluated radiant energy.
• The visible portion of the radiant energy that reaches the eye is absorbed by special receptors (rods and cones) in the retina, which covers the inner wall of the eye.
• In the retina, the rods and cones convert the radiant energy into electrical signals. The nerves transmit the electrical impulses to the brain where the light sensation is created.
Spectral Sensitivity of the Eye
• The sensitivity of the human eye is not uniform over the visible spectrum. Different wavelengths give different colour impressions and different brightness impressions.
• The “relative spectral luminous efficiency curves” (shown on the next page) give the ratio of the sensitivity to each wavelength over the maximum sensitivity.
• The curve for photopic (or day) vision applies when the eye is in bright viewing conditions. The curve is denoted by V (_). The visual response is at maximum at the yellow green region of the spectrum, at a wavelength of 555 nm.
• The curve for scotopic (or night) vision applies when the eye is in dark-adapted condition. The curve is denoted by V’ (_).
The visual response is at maximum in the blue-green region of the spectrum, at a wavelength of 507 nm.
Relative Spectral Luminous Efficiency Curves

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c. Spectral Power Distribution
Introduction
• Each light source is characterized by a spectral power distribution curve or spectrum.
• Spectral Power Distribution Curve
• The spectral power distribution (SPD) curve, or spectrum, of a light source shows the radiant power that is emitted by the source at each wavelength, over the electromagnetic spectrum (primarily in the visible region).
• With colour temperature and colour rendering index ratings, the SPD curve can provide a complete picture of the colour composition of a lamp’s light output.

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Incandescent Lamp Spectrum
• Incandescent lamps and natural light produce a smooth, continuous spectrum.
High Intensity Discharge Lamp Spectrum
• HID lamps produce spectra with discrete lines or bands.
Fluorescent Lamp Spectrum
• Fluorescent lamps produce spectra with a continuous curve and superimposed discrete bands.
• The continuous spectrum results from the halophosphor and rare earth phosphor coating.
• The discrete band or line spectrum results from the mercury discharge.
d. Lighting and Colour
Introduction
• Each wavelength of light gives rise to a certain sensation of colour.
• A light source emitting radiant energy, relatively balanced in all visible wavelengths, such as sunlight, will appear white to the eye.
• Any colour can be imitated by a combination of no less than three suitable primary colours.
• A suitable set of primary colours usually chosen is red, green and blue.
• A beam of white light passing through a prism is dispersed into a colour spectrum.

Optical Prism

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Surface Colours
• The perceived colour, or colour appearance, of a surface is the colour of the light reflected from the surface.
• Certain wavelengths are more strongly reflected from a coloured surface than others, which are more strongly absorbed, giving the surface its colour appearance.
• The colour depends on both the spectral reflectance of the surface and the spectral power distribution of the light source. In order to see the colour of the object, that colour must be present in the spectrum of used light source.
Colour Properties of Light Source
• The colour properties of a light source depend on its spectral power distribution.
• The colour properties of a light source are described by three quantities:
-Chromaticity - or colour temperature (CT)
-Colour rendering index
-Efficiency (lumen/watt)
Chromaticity or Colour Temperature
• All objects will emit light if they are heated to a sufficiently high temperature.
• The chromaticity or colour temperature of a light source describes the colour appearance of the source.
• The correlated colour temperature of a light source is the absolute temperature, in Kelvin (K), of a black-body radiator, having the same chromaticity as the light source.
• Sources with low colour temperatures - below 3,000 K have a reddish or yellowish colour, described as warm colour.
• Sources with high colour temperatures - above 4,000 K have a bluish colour, described as cool colour.
• Warm colour is more acceptable at low lighting levels and cool colour at high lighting levels.
• The colour description and application is summarized as follows:
below 3,000 K warm reddish lower lighting levels
above 4,000 K cool bluish higher lighting levels.
Colour Temperature of Common Light Sources
Colour Temp
Light Source (K) Description
Sky - extremely blue 25,000 25,000 cool
Sky - overcast 6,500 cool
Sunlight at noon 5,000 cool
Fluorescent - white 4,100 cool
Metal halide (400 W, clear) 4,300 cool
Fluorescent - white 3,000 warm
Incandescent (100 W) 2,900 warm
High Pressure Sodium (400 W, clear) 2,100 warm
Candle flame 1,800 warm
Low pressure sodium 1,740 warm

Colour Rendering Index (CRI)
• Colour rendering is a general expression for the effect of a light source on the colour appearance of objects, compared with the effect produced by a reference or standard light source of the same correlated colour temperature.
• The colour rendering properties of a light source are expressed by the (CRI).
• The CRI is obtained as the mean value of measurements for a set of eight test colours.
• The CRI has a value between 0 and 100.
• A CRI of 100 indicates a light source, which renders colours as well as the reference source.
• The CRI is used to compare light sources of the same chromaticity (or colour temperature).
• The CRI is used as a general indicator of colour rendering: a higher CRI means a better colour rendering.
• It is essential to understand that the CRI value has no reference to ‘natural’ light, although colours under a high CRI lamp will appear more natural.
• The most important characteristic of a lamp, from an energy viewpoint, is its ability to convert electrical energy into light. This measure is referred to as efficacy, in lumens per watt or light output per watt input. The chart below shows the general range of lumens per watt and the CRI for various light sources.
Colour Rendering Index and Efficacy of Common Light

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Technology and Performance
• Incandescent lamps produce smooth, even SPD curves and outstanding CRI values.
• Halogen versions of incandescent lamps produce whiter light with +95 CRI.
• With gaseous discharge technology, colour characteristics are modified by the mixture of gases and by the use of phosphor coatings.
• HID lamps are chosen mostly for their exceptional energy efficiency; metal halide versions have acceptable CRI levels.
Application Notes
• Warm colour light is associated with indoors, nighttime and heat, and fits better indoors and in cool environments.
• Warm colour light makes warm colour objects (red-yellow colours) look richer.
• Cool colour light is associated with outdoors, daytime and cold, and fits better in warm environments.
• Cool colour light mixes better with daylight (daytime lighting)
• Cool colour light makes cool colour objects (blue-green colours) look refreshing.
• Match light source colour with room objects’ colour (interior decoration).
• Sources with high CRI cause the least emphasis or distortion of colour.
e. Lighting Quantities and Units
Luminous Flux or Light Output
• The luminous flux, or light output, is defined as the total quantity of light emitted per second by a light source.

• Sensitivity of the human eye varies, reaching its maximum at a wavelength of 555 nm during daytime (photopic vision) and 507 nm for night vision (scotopic vision)
• The unit of luminous flux is the lumen (lm).
• The lumen is defined as the luminous flux associated with a radiant flux of 1/683 W at a wavelength of 555 nm in air.
• Lamp Lumens (lm) = the quantity of light emitted by a light source.
Luminous Efficacy
• The luminous efficacy of a light source is defined as the ratio of the light output (lumens) to the energy input (watts).
• The efficacy is measured in lumens per watt (lm/W).
• The efficacy of different light sources varies dramatically; from less than 10 lumens per watt, to more than 200 lumens per watt.
• Efficacy of a light source = lamp lumens/lamp watt

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Luminous Intensity
• Luminous intensity is the amount of light emitted in a certain direction
•Luminous intensity refers to the directional luminous flux emitted per steradian
•The unit of luminous intensity is the candela (cd)
•A steradian is the unit for a solid angle. A solid angle is the measure of a space around a point
•Luminous intensity is an important concept
Source: Philips Lighting http://www.lighting.philips.com.ph/v2/knowledge/basics.jsp?id=149287

Luminous Flux Density or Lighting Level
• The luminous flux density at a point on a surface is defined as the luminous flux per unit area.
• The luminous flux density is also known as the illuminance, or quantity of light on a surface, or lighting level.
• The SI unit of the lighting level is the lux (lx), 1 lx = 1 lm/m2.
• When measurement is in Imperial units, the unit for the lighting level is the foot candle (fc): 1 fc = 1 lm/ft2.
• The relation between the fc and lux is 1 fc = 10.76 lux.
Incidentally, this is the same as the relationship between square meters and square feet. : 1 m2 = 10.76 ft2.
• The lighting level is measured by a photometer, as shown in the figure below.
• Minimum recommended lighting levels for different tasks are included below.
• Lux = the unit of illuminance at a point of a surface.
• Lux = lumens/area.

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f. Lighting Levels
Introduction
• Recommendations for lighting levels are found in the 9th Edition of the IESNA Lighting Handbook. The
Illuminating Engineering Society of North America is the recognized technical authority on illumination.
• The data included in the tables below is approximate and describes typical applications.

Lighting Levels by Visual Task

Type of Visual Task fc lux Comments
TASKS OCCASIONALLY PERFORMED 3 30 ORIENTATION & SIMPLE VISUAL TASKS
SIMPLE ORIENTATION/SHORT VISITS 5 50 ORIENTATION & SIMPLE VISUAL TASKS
WORKING SPACES/SIMPLE TASKS 10 100 ORIENTATION & SIMPLE VISUAL TASKS
HIGH CONTRAST/LARGE SIZE 30 300 COMMON VISUAL TASKS
HIGH CONTRAST/SMALL SIZE OR INVERSE 50 500 COMMON VISUAL TASKS
LOW CONTRAST/SMALL SIZE 100 1,000 COMMON VISUAL TASKS
TASKS NEAR THRESHOLD 300-1,000 3,000-10,000 SPECIAL VISUAL TASKS

Examples of Lighting Levels by Building Area and Task

Lighting Level
Building Area and Task fc lux Comments
AUDITORIUMS 10 100 INCLUDE PROVISION FOR HIGHER LEVELS
BANKS - TELLERS’ STATIONS 50 500
BARBER SHOPS 50 300
BATHROOMS 30 300
BUILDING ENTRANCES (ACTIVE) 30 50
CASHIERS 30 300
CONFERENCE ROOMS 30 300 PLUS TASK LIGHTING
CORRIDORS 5 50
DANCE HALLS 5 50
DRAFTING - HIGH CONTRAST 50 500
DRAFTING - LOW CONTRAST 100 1,000
ELEVATORS 5 50
EXHIBITION HALLS 10 100 INCLUDE PROVISION FOR HIGHER LEVELS
FLOODLIGHTING - BRIGHT 5 50 LESS FOR LIGHT SURFACES – MORE FOR DARK
SURROUNDINGS (VERTICAL)
HOSPITALS - EXAMINATION ROOMS 50 500 HIGH COLOUR RENDITION
HOSPITALS - OPERATING ROOMS 300 3,000 VARIABLE (DIMMING OR SWITCHING)
Kitchen 50 500
Laundry 30 300
Lobbies 10 100
Office - General 30 300
Parking Areas - Covered 0.2 20 Lower at night
Parking Areas - Open 0.2 2 Higher for enhanced security
Reading/Writing 50 500 Varies with task difficulty
Restaurant - Dining 10 100
Stairways 5 50
Stores - Sales Area 30 300
Streetlighting - Highways 0.9 9 Varies with traffic density
Streetlighting - Roadways 0.7 7 Varies with traffic and pedestrian density

Lighting Level Adjustment

Reduce Lighting Increase Lighting
Factor Level by 30% Level by 30%
Reflectance of task background Greater than 70% Less than 70%
Reflectance of task background Greater than 70% Less than 70%
Reflectance of task background Greater than 70% Less than 70%