Chapter 8 Quantity of Light

What are the primary ways in which we consider light output from a source:

1. Lumen Unit used to measure light output from a source 2. Candela Unit used to measure light output in a specific direction from a source

What is the average illuminance on a work surface?

30 fc; determined by the following first 2 steps

Point Method #3 Horizontal surfaces: Intensity (candelas) = (Illuminance level x distance squared) / cosine of the angle

Accent fixture recessed into 10' ceiling. Directly below is a plate on a pedestal at 36" above the floor. Fixture is angled to aim at the plate. You want to illuminate the plate to 100 foot-candles What CBCP value would be needed from the luminaire? Illuminance = _______ D = 7.5 ft D2 = ______ Cosine (30°) = .866 Candelas = ___

What is the light loss factor?

Accounts for depreciation of lighting system over time • Lamp lumen depreciation (LLD) • Luminaire dirt depreciation (LDD) • Room surface dirt depreciation • Lamp burnout • Lamp position tilt factor • Ballast factor Standard assumption of 0.85 Only 85% of the rated lumen output will actually make it out of the light source Calculation can be shortened to LLD x LDD = LLF

What is Lumination Dirt Depreciation?

Accounts for loss of light caused by dirt and dust accumulation.

How is light essential to sustainable environments

By reducing the quantity of lighting and flexing illumination levels

Point Method #3 on horizontal surfaces

Candela value = (Illuminance level x distance squared) / cosine of the angle Vertical surface: use sine instead of cosine

What id Photometric data?

Candlepower distribution curves are used to determine the direction, pattern, and intensity of light from reflector lamps and luminaires.

Point Method #3

Deducing CBCP when lighting is not perpendicular to the object/surface

What is Illuminance (E)

Density of luminous flux received by a surface, or total amount of light falling on a surface Measured in fc (lumens per square foot) or lux (lumens per sq meter)

Calculate Point-by-point method

Determines the light level for specific task, accent or object lighting Inverse square law = the level of illumination on a surface decreases the farther a surface is from the light source Light spreads out as it travels away from the source

Point Method #3 Intensity (candelas) = (E x D2) / cosine of angle

E = 100 fc D = 7.5 ft D2 = 56.25 sq. ft. Cosine of 30° angle = .866 Candelas = (100 fc x 56.25) / .866 Candelas = 6,495

How do you determine point by point angled on a vertical surface?

E+ I/D2 x sin 0 degree

How do you determine point by point angled on a horizontal surface?

E- I/D2 x cos 0 degree

Point Method #1 Illuminance (E) = I / D2

I = 10,000 candelas D = 10ft - 2,5 = 7.5 D2 = 56.25 Illuminance = 10,000 / 56.25 Illuminance = 178 fc

Point Method #2

I = E x D2 E = 100 fc D = 7.5 ft D2 = 56.25 sq ft Candelas = 100 fc x 56.25 sq ft Candelas = 5,625

How to calculate using method 1

Illuminance (E) = lumens / area (sq. ft.) • Area = 10'x15' = 150 sq ft • Lumens = 1,000 lms x 5 lights = 5,000 • Illuminance = 5,000 lms / 150 sq. ft • 33.3 lms per sq. ft. • Illuminance (E) = 33.3 FC

How do you do Lumen calculations? Method #1

Lumen Method #1: Determining illuminance level How many foot-candles (illuminance) are in this room? Illuminance (E) = lumens / area (sq. ft.)

What is calculation method #2

Lumen Method #2: Determine lumens and # of luminaires How many lumens are needed to reach the desired illuminance level? Lumens = Illuminance x area (sq. ft.)

How to calculate using method #2

Lumens = Illuminance (E) x area (sq. ft.) • Area = 10'x15' = 150 sq ft • Illuminance = 60 fc • Lumens = 60 fc x 150 sq ft • Lumens = 9,000 How many luminaires are needed?

Show one standard luminaire

Luminaire type CU% Direct fixture 85% Indirect Fixture 50% Spot/accent 95% Wash/ambient 75%

How to translate lumens vs. lumnaires:

Luminaires = lumens required / lumens per luminaire A luminaire outputs 550 lumens 9,000 lumens / 550 = __________ (# of luminaires) If multiple lamps per luminaire, determine lumens per luminaire: Lumens per lamp x Lamps per luminaire = Lumens per luminaire 550 lumens per lamp x 2 lamps per luminaire = 1,100 lumens per luminaire 9,000 lumens / 1,100 = ___________ (# of luminaires)

What is Photometry?

Measurement of visible light - includes the human response to a source of illumination

What is luminous exitance? (M)

Measures total quantity of luminous flux leaving a surface in all directions; measured in lumens per sq ft or lumens per sq meter

What is Lamp Lumen Depreciation?

Metric measurement of the loss of lumens due to the design of a lamp.

What is point by point method used for?

Mostly used for lighting specific surfaces from a certain distance by a directional fixture - object, accent and task lighting

What is the next step in the Point-by-Point Method?

Next step: Find the lamp or luminaire(s) that will produce the required candelas Technical specs from lighting manufacturers will list CBCP values produced by lamps/luminaires

Photometric Data Chart

Photometric chart of a linear direct luminaire with a parabolic louver.

What is Room Cavity Ration?

RCR -A method of specifying room proportions and distance from luminaires to work surface Used to help determine the Coefficient of Utilization What RCR means: Low and wide: RCR=1 Cube : RCR = 10 High and narrow: RCR = 20

CBCP cut sheet

Some manufacturers will list CBCP on the cut sheet

Show point by point method

Source aimed directly at a surface. Illuminance (E) = Luminous Inttensity (I) Distance squared (d2). E = I/D2

What is luminous intensity?

The intensity of the luminous flux forced from a light source in a given direction; measured in candelas (cd)

What is Luminance?

The measure of the objective brightness of a light source, or what is actually received by the eye of the viewer

What is Radiometry?

The measurement of radiant energy in the form of electromagnetic waves

What is the measure of lighting based upon?

The principles of radiometry and photometry

What is the coefficient of utilization? (CU)

The ratio of lamp lumens which ultimately reach the work plane to the total lumens generated by the lamp Used to help determine the average foot-candle level (illuminance) CU accounts for: • Direct light - from luminaires • Interreflected light - from walls, ceiling, and floor Considers room geometry, fixture orientation, surface reflectance Each surface reflects some light from the luminaire onto other surfaces with each surface ending up with more light (or maybe less) on it

What is luminous flux?

The total amount of illumination emitted by a light source; measured in lumens (lm)

Understanding calculations is critical in what way?

Understanding how lighting calculations are performed is critical to knowing the variables that affect the quantity of light on a work plane and in a space.

How is the Lumen method used?

Used to determine the average illuminance on horizontal surfaces in large open areas. Used for ambient levels Intended to tell you about a specific surface, such as the floor or a task plane in a room

Point Method #2 Determine candela value needed for a certain lighting task - will help to find an appropriate lighting source Intensity (I) = Illuminance level (E) x distance squared (D2) I = E x D2

ntensity (I) = Illuminance level (E) x distance squared (D2) • Accent fixture recessed into 10' ceiling • Directly below is a plate on a table at 2'-6" above the floor • You want to illuminate the plate to 100 foot-candles • What CBCP value would be needed from the luminaire Illuminance = 100 fc Distance = ______ D2 = ______ Candelas = ______

How do you determine the spacing criteria and intervals for lamps using photometric data?

the formula for calculating the center- to- center distance between fixtures is: SC ratio x mounting height (MH - distance from the bottom of the luminaire to the work surface) - spacing intervals (SI)

What does one candela represent?

the luminous intensity from a source pointing in a specific direction on a solid angle called the steradian.

Point Method #1 Determine illuminance level of a source onto a point Illuminance (E) = I / D2

• Accent fixture is recessed into 10' ceiling • A plate on a table at 2'-6" above the floor • CBCP = 10,000 candelas • What is the illuminance level (foot-candles) at the center of the plate? Candelas = 10,000 Distance = ______ D2 = ______ Illuminance = ______

What does each light source have?

• one total lumen output value and • a different candela value for output in every direction