Light Quantity
Glare vs. Sparkle
Lighting Metrics
Luminous Flux
Illuminance
Luminous Intensity
Luminance
Luminance Exitance
Glare
 More light is not better.
 Uncontrolled bright light in the field of
view is glare.
Glare
 Glare can also be experienced as
Veiling Reflection
Glare
 Knowing the direction of light can
reduce the possibility of glare.
Sparkle
 Sparkle is uncontrolled light that is not
visually distracting.
Lighting Metrics
Luminous Flux or how much light does
that lamp (light bulb) produce?
watts
Notice Lumens
840 Lumens
Luminous Flux
Energy In Watts
When energy
(electricity) flows into
a lamp it is converted
into light. The total
amount of light that
flows from the lamp
is Luminous Flux.
The measure of
Luminous Flux is
Lumens.
Energy Out Lumens (Light)
Luminous Flux or how much light does
that lamp (light bulb) produce?
(PHI)

= Luminous Flux
The measurement of
Luminous Flux is Lumens
The 60 Watt Soft White Lamp
Produces 840 Lumens
 = 840 Lumens
Luminous Flux
If you are looking for the lamp that
produces the most amount of light
you look for the one with the most
lumens.
Designers must also be concerned
with the energy efficiency of light.
A lamp that produces a small amount
of light and uses a lot of energy is not
efficient.
With everything else equal, you want
to get the most amount of light for the
least energy. Energy=$$
Luminous Flux
By dividing the number of lumens
produced by the amount of energy
used you can determine the
energy efficiency or efficacy of the
lamp.
Efficacy =
 (Lumens)
Watts
What is the Efficacy of the Above Lamp?
Luminous Flux

= 840 Lumens
Watts = 60
Efficacy =
840 Lumens
60 Watts
Efficacy = 14 Lumens/Watt*
For every watt of energy used this lamp produces 14 lumens.
*Always use nomenclature in all calculations.
Luminous Flux
We can use this calculation (efficacy) to compare different
types of lamps to compare energy efficiency.
The rated lumens of lamps can be found in lamp manufacturer
catalogs. Lamp catalogs can be downloaded at:
General Electric (general web site www.gelighting.com)
Philips (general web site www.philips.com)
Sylvania (www.sylavania.com)
Luminous Flux
Compact Fluorescent
Watts = 14 Lumens =745
Efficacy =
745 Lumens
14 watts
Efficacy = 53.2 Lumens/Watts
Why Mean Lumens?
Some lamps create
more light when they are
new. The mean number
of lumens takes into
account the light loss
over time.
Luminous Flux
HID Metal Halide
Watts = 100 Lumens
Efficacy =
=6200
6200 Lumens
100 watts
Efficacy = 62 Lumens/Watts
Luminous Flux
Summary of Efficacy
60 Watt ‘A’ Incandescent = 14 Lumens/Watt
14 Watt Compact Fluorescent = 53.2 Lumens/Watt
100 Watt HID Metal Halide = 62 Lumens/Watt
The fluorescent is 3.8 times more energy efficient than the incandescent
and the HID lamp is 4.4 time more energy efficient than the
incandescent.*
*Efficacy is just one of many factors to be taken into account when
comparing lamps. CCT, CRI, and the ability to optically control the lamp
are other important considerations.
Illuminance
How much light do you need to eat pizza, read a newspaper, or prepare
food? All these tasks require different amounts of light to complete. You
know that you need more light to read a newspaper than to eat pizza. But
do you need more or less light to cook a pizza than read a newspaper?
These questions are ones which ask how much light falls on a surface.
The surface may be a pizza, a newspaper, or the counter in a kitchen.
When you want to measure the amount of light needed to complete a
task, you will need to understand illuminance.
Illuminance
The light that strikes an
object is illuminance. In the
illustration to the right, the
light is striking the light
meter. The light meter
measures illuminance levels.
The units for illuminance are
footcandles. (fc).
As a designer, you will be
designing spaces with
different tasks. Each of these
tasks will require different
illuminance levels.
Illuminance
To determine recommended Illuminance levels IES
(Illuminating Engineering Society) researches and provides
guidelines.
You can find more information IES at:
http://www.iesna.org/
Illuminance
The guidelines start with broad categories that range
from general lighting through detailed tasks.
Lux = Lumens per square meter.
Footcandles = Lumens per square foot.
6 - Provide Higher level over food
service or selection areas.
Illuminance - IES Recommendations
Eating Pizza = Illuminance Category B (5-7.5-10 fc)
Reading Newspaper = Illuminance Category D (20-30-50 fc)
Cooking a Pizza = Illuminance Category E (50-75-100 fc)
Illuminance
Illuminance can be expressed as
the following:
E = Illuminance
 (incident luminous flux*)
E=
Area
(square feet)
*Incident luminous flux is the flow of light
that strikes a surface.
Illuminance
If 1500 lumens fall on a desk that is 36” X 60”
what is the illuminance level on the desk?

(Incident luminous flux) = 1500 lumens
Area = 3’ x 5’ = 15 square feet
E=
E=

Area
1500 Lumens
15 square feet
= 100 fc
Luminous Intensity
Luminous Intensity is the force generating the luminous flux, or the force
behind the flowing light.
An analogy is water flowing from a hose. The pressure of the water in the
hose is similar to luminous intensity. It is important to consider both the
direction and force when considering luminous intensity.
Area = 3' * 5' = 15 square feet
Luminous Intensity
To represent the intensity and the direction of light we use a candle power
distribution curve or intensity distribution diagram.
Area = 3' * 5' = 15 square feet
The candle power distribution diagram is typically found in light fixture
photometric data. The light fixture modifies the direction and intensity of
light to aid in the reduction of glare and efficiency.
Luminous Intensity
The candle power distribution
diagram illustrates both the
intensity and direction of light from
a light fixture.
The center of the diagram aligns
with the center of the light fixture.
Area = 3' * 5' = 15 square feet
The solid dark lines indicate the
direction and the intensity of the
light flowing from the fixture.
(Similar to water flowing from a
hose.)
Light from this fixture flows both up
and down.
Luminous Intensity
UP
How many candelas are
being emitted from the light
fixture at 45 Degrees?
600 Candelas
Center of light fixture
Candelas
Light Direction and Intensity
Direction
(Thick black line)
DOWN
Luminous Intensity
Analyze the fixture compared to the
direct glare zone. Does the fixture
produce light in the undesirable
zone?
NO
The fixture also distributes the light in a wide angle upward
which reduces the potential glare or hot spots on the ceiling.
Luminous Intensity
What you can understand from
candela distribution diagrams.
1. Direction of Light (wide beam,
narrow beam, up, down, up
and down)
2. Intensity of Light in Candelas*
in Any Given Direction
(candelas and lumens are
interchangeable units)
3. Potential for Glare
Luminous Intensity - Inverse Square Law
We know through experience that as a light source moves
further away from a given surface the intensity of light on
that surface is reduced.
The relationship between light intensity and
distance can be described in the inverse
square law / point calculation method.
Luminous Intensity - Inverse Square Law
The equation for the inverse square law is as follows:
E = Illuminance (fc)
I
E=
I / C = Candelas (cd)
D2
D = Distance
Luminous Intensity - Inverse Square Law
What is the
Illuminance of a
surface that is
illuminated with a 800
lumen lamp at a
distance of 1’?
E=
I
D2
I = Candelas = 800 cd
D = Distance = 1'
800 Candelas
E=
= 800 footcandles
2
1'
Luminous Intensity - Inverse Square Law
What is the
Illuminance of a
surface that is
illuminated with a 800
lumen lamp at a
distance of 2'?
E=
I
D2
I = Candelas = 800 Candelas
D = Distance = 2'
800 candelas
E=
= 200 footcandles
2
2'
Luminous Intensity - Inverse Square Law
What is the
Illuminance of a
surface that is
illuminated with a 800
lumen lamp at a
distance of 4'?
E=
I
D2
I = Candelas = 800 Candelas
D = Distance = 4'
800 Candelas
E=
= 50 footcandles
2
4'
Luminous Intensity - Inverse Square Law
When the distance of a light doubles from the surface it is
illuminating the amount of light decreases by a factor of 4.
800 footcandles
@ 1'
200 footcandles
@ 2'
50 footcandles
@ 4'
Luminous Intensity - Inverse Square Law
8'-6"
We may need to
rearrange the formula
to make it practical
for interior designers.
5'-0"
1'-6"
A task on a desk requires 75 footcandles. You can choose to
illuminate the task with a desk lamp (portable fixture), a
pendent fixture, and/or a surface mounted fixture. Calculate
the amount of lumens required for each lamp based on the
placements illustrated above.
Luminous Intensity - Inverse Square Law
8'-6"
5'-0"
1'-6"
E=
I
D2
E = Illuminance = 75 footcandles
D = Distance = 8'-6"
I = D2 X E
I = 8.5'2 X 75footcandles = 5,419 candelas
Luminous Intensity - Inverse Square Law
8'-6"
5'-0"
1'-6"
E=
I
D2
E = Illuminance = 75 footcandles
D = Distance = 5’-0”
I = D2 X E
I = 5’2 X 75footcandles = 1,875 candelas
Luminous Intensity - Inverse Square Law
8'-6"
5'-0"
1'-6"
E=
I
D2
E = Illuminance = 75 footcandles
D = Distance = 1’-6”
I = D2 X E
I = 1.5’2 X 75footcandles = 169 candelas
Luminous Intensity - Inverse Square Law
By using lamp catalogs one can
find lamps to match the candelas
solved with matching center beam
candle power ratings.
The Center Beam Candle Power
(CBCP) is the approximate
amount of candelas at 0 degrees
to the light source. (The light
shining straight down.)
These ratings are given for
directional lamps such as PAR, R,
and MR16. (Spots and Floods)
0
Luminous Intensity - Inverse Square Law
Find a lamp near 5,419 CBCP. (Center
Beam Candle Power)
Find a lamp near 1,875 CBCP. (Center
Beam Candle Power)
Luminous Intensity - Inverse Square Law
Find a lamp near 169 Center Beam Candle
Power.
Luminous Intensity - Inverse Square Law
To generate 75
footcandles:
The ceiling mounted
fixture requires 6 times
the energy compared to
the desk lamp.
The pendent fixture
requires 2 times the
energy compared to the
desk lamp.
150 watt
Incandescent
45 watt
Incandescent
27 watt
Incandescent
Luminous Intensity - Inverse Square Law
3'
The inverse square law as
stated below only works for a
light that is at 90 deg to the
surface it is illuminating.
E=
I
D2
D = Distance = 3'-0"
I = 500 Candelas
E=
500 Candelas
9 sq. ft.
E=55.5 Footcandles
But what if the light is at an
angle to the surface?
Luminous Intensity - Inverse Square Law
3'
3' 60 deg
The amount of light that reaches a
surface is reduced when it is at an
angel to the plane that it is striking.
The amount of reduction is equal
to the cosine of the angle.
I
E= cos angle
D2
D = Distance = 3'-0"
I = 500 Candelas
Cos 60 deg = .50
E= .50
500 Candelas
9 sq. ft.
E=28 Footcandles
Lighting Calculations
 Lumen Method – determines average light
levels in large open areas
 Point Calculations – determines light
levels at a specific point on an object or
surface
Lumen Method Calculations
 1 foot candle (FC) = 1 Lumen (Lms)
divided by 1 square foot (area / A)
 1 FC = 1 LM ÷ A
 Illuminance (E) (amount of light falling onto
an object measured in foot candles) =
Lumens divided by Area (in sq. ft.)
 E = Lms ÷ A (sq. ft.)
Lumen Method Example 1
Solving for Illuminance(E) Level
 Room 10’ x 15’
 5 down lights
 1000 Lumens each down light
 E = Lms ÷ A






Illuminance = Lumens divided by Area
A = 10’ x 15’ = 150 sq. ft.
1000 Lumens x 5 down lights = 5000 lumens
E = 5000 Lms ÷ 150 sq. ft.
E = 37.5 Lms per sq. ft.
E = 37.5 foot candles (FC)
Lumen Method Example 2
Solving for Lumens or Luminaires needed





Room 10’ x 15’ (A = 150 sq. ft.)
Illuminance level (E) = 60 foot-candles
Need Lumens / Luminaires
E = Lms ÷ A - original equation
Lumens needed = Illuminance x Area
 Lms = E x A
 Lms = 60 E x 150 sq. ft.
 Lms = 9000 Lumens
Lumen Method Example 2
Solving for Lumens or Luminaires needed
 Lumens needed = Illuminance x Area
 Lms = E x A
 Lms = 60 E x 150 sq. ft.
 Lms = 9000 Lumens
 550 lumens = per luminaire
 Luminaires needed = Lumens needed divided by
Lumens per luminaire
 Luminaires (X) = 9000 (lumens) ÷ 550 Lumens per
Luminaire
 9000 ÷ 550 = 16.36
 Always round up with any point.
 Luminaires needed = 17
Lumen Method Safety Factors
 Light loss factor (LLF) – light loss due to
deteriorating lamps
 Industry standard of .85 for a light loss factor
 Coefficient of Utilization (CU) – effectiveness
of luminaires working on surfaces of the
space to deliver light to the target surface.




Direct fixture or downlight – CU = 85% (0.85)
Indirect fixture – CU = 50% (0.50)
Spot or Accent – CU = 95 (0.95)
Wash or ambient – CU = 75% (0.75)
Lumen Method Calculations with
Safety Factors
 Illuminance = Lumens ÷ Area
 Illuminance = (Lumens x LLF x CU) ÷
Area
 Lumens needed = Illuminance target x
Area
 Lumens needed = (Illuminance target x
Area) ÷ (LLF x CU)
The Point Calculation Method
 Estimate location of lighting equipment
 Light density = Candle power = Candela
 Candela information given by
Manufacturer
 Center Bean Candle Power – CBCP –
center of a directional light
 Illuminance = Candela value ÷ Distance
(squared)(in feet)
 E = CD ÷ D2
Point Calculation Example 1
 10’-0” ceiling
 Accent Fixture
 Above a plate on a table 3; above the
finished floor (A.F.F.)
 Illuminance = Candela value ÷ Distance2





E = CD ÷ D2
CD = 10,000 Candelas
D2 = 7 ft 2 = 49 square feet (units get squared too)
E = 10,000 CD ÷ 49 sq.ft.
E = 204 Foot-candles
Point Calculation Example 2
 10’-0” ceiling
 Accent Fixture
 Above a plate on a table 3; above the finished
floor (A.F.F.)
 Candela value needed = Illuminance level desired
x distance squared





CD = E x D2
E = 150 Foot-candles
D2 = 7 ft2 = 49 sq. ft.
CD = 150FC x 49 sq. ft.
Candela value needed = 7350 Candelas
Luminaire and Lamp Type
 Candela represented in form of a distribution
diagram
 Provided by manufacturers
 When sources is not perpendicular (90
degree angle to surface)
 Candela value needed = (Illuminance level
desired x distance squared) ÷ cosine of the angle
 CD = (E x D2) ÷ cosine of angle
 E = (CD x cosine of angle) ÷ D2
Point Calculation Example 3





10’-0” ceiling
Accent Fixture
Above a plate on a table 3’ above the finished floor (A.F.F.)
30 degree angle for fixture
100 foot-candles
 Candela value needed = (Illuminance level desired x distance
squared) ÷ cosine of angle
 CD = (E x D2) ÷ cosine of angle
 E = 100 FC (foot-candles)
 D2 = 7’ feet = 49 sq. ft.
 Cosine of angle = cosine of 30 degrees = 0.87
 CD = (100 FC x 49 sq. ft.) ÷ 0.87
 Candela value needed = 5633 Candelas.
Photometric Data
Photometry is the science of measurement
of light.
Photometric data is a set of information
related to the amount of light emitted from
individual fixtures.
Independent labs are used to verify and
objectively report photometric data from
fixture manufactures
Luminance
The luminance is only one of two metrics that can be seen by
an observer.
Luminance is the intensity of light on the surface on an object
emitting light.
When a lamp is on and you are looking at the lamp you see
luminance. Other examples include neon signs, and soda
machines.
Luminance
Luminance can provide
dramatic visual interest,
focus and or direction in
a space.
But luminance can also be the cause of direct glare in
light fixtures. If you can see the lamp in a light fixture that
is luminance.
Luminance
Luminance can be understood as the amount of candelas
per square inch on the surface of the object emitting light.
Luminance=250 cd/sq.in.
When the object emitting light is viewed at an angle the
luminance level is reduced. The factor that the luminance
level is reduced is the cosine of the angle.
Luminance=250 cd/sq.in. X cos 40 degrees
40 deg.
cos 40 deg. = .77
Luminance=250 cd/sq.in. X .77
Luminance=192.5 cd/sq.in.
Luminance Exitance
Luminance exitance can also be seen by an observer.
It is the luminance flux that is leaving a surface after it has
been reflected from a material.

E
Luminance Exitance can be
expressed as:
M=
 XE
M= Luminance Exitance (footcandles)
 (rho) = Reflectance Value
E = Illuminance (footcandles)
Luminance
Exitance
Luminance Exitance
The reflectance value is the percentage of light reflected
by a material.
 (rho) = Reflectance Value

E
Light colors such as white can reflect upwards
of 90% of the light.
Dark colors such as black absorb up to 85%
of the light.
All materials absorb light energy, thus the
reflectance value will always be less than
(1.0).
Luminance
Think of reflectance values are like taxes they
Exitance (M) take away, never add.
Luminance Exitance
Every material has a reflectance value. You can find the
reflectance value of paints on the back of the paint chip.
Manufactures of materials typically publish the reflectance
values in the material specifications.
Wall Coverings
Ceiling tiles/systems
Paints
Flooring (carpet and tile)
If you can not find a published reflectance value you can
use paint chips to match the value. Use the published
reflectance value of the matching paint for the unknown
reflectance value.
Luminance Exitance
If a wall is painted white and 100 footcandles are striking
the wall how much light is being reflected by the wall?
 (rho) = Reflectance Value = .88 (88%)

E
E = Illuminance = 100 fc
M=
 XE
M = .88 X 100 fc = 88 fc (footcandles)
Luminance
Exitance (M)
What is the luminance exitance of a wall that
has a reflectance value of 50% and is receiving
40 foot candles?
Luminance Exitance
What is the luminance exitance of a wall that has a
reflectance value of 50% and is receiving 40 foot candles?
 (rho) = Reflectance Value = .5 (50%)

E
E = Illuminance = 40 fc
M=
 XE
M = .50 X 40 fc = 20 fc (footcandles)
Why is luminance exitance important?
Luminance
Exitance (M)
Luminance Exitance
When light a light fixture is used to illuminate a room some
of the light reaches the table directly, and some light
reaches the table by reflecting off of the walls.

The higher the reflectance value of the
surfaces in the room the more light that is
reflected onto the table.
This increased the energy efficiency of
the lighting system.
When calculating the number of fixtures
required for a room you use the
reflectance values of the room in the
calculation. (Dark materials in the room
more fixtures, light materials in the room
less fixtures.)
Luminance Exitance
When using a indirect light fixture it is important that you
use a ceiling with a high reflectance value.

The ceiling in an
indirect lighting
system is part of
the light fixture.
The ceiling must
reflect most of the
light for the
lighting system to
be efficient.
Luminance Exitance
Due to selective absorption, the reflected light will take on
the color of the surface of the wall. A red wall will reflect
red light onto a white table surface.

The table top will most likely show up as
pink, a tint of red, because it is mixing
with some of the direct light from the light
fixture.
Not just walls, but any large surfaces in a
room, will reflect the color back onto
other surfaces in a space. A white ceiling
may take on the color of the flooring
through reflected light.
Luminance Exitance
A high reflectance value is not always desirable. Sometimes it
is important to control light by limiting reflection.
Luminance Exitance
Low light reflectance on
the ceiling (a dark ceiling)
creates a more intimate
atmosphere.
Luminance Exitance
High
reflectance
values
(lighter
spaces) tend
to visually
expand
space.
Photometric Data
Photometric Data may include the following
information:
Candle Power Distribution Curve
Luminaire Efficiency
Coefficient of Utilization
Spacing Criterion
Physical Dimensions
Lamp and Ballast Data
Efficacy
Photometric Data
Candle Power Distribution Curve
Describes the direction and the intensity of light
from the luminaire.
Candle power distribution curves can also help
determine if the fixture may create problems
with direct and reflected glare as well as
problems with hot spots.
Photometric Data
Candle Power Distribution Curve
‘Batwing’ Distribution (Good for Computers)
Photometric Data
Candle Power Distribution Curve
‘Even’ Distribution
Photometric Data
Candle Power Distribution Curve
Potential Problems:
Photometric Data
Indirect bat wing patter
may reduce hot spots
(unacceptable bright
areas) on the ceiling.
Semi Indirect with a bat
wing patter down to
reduce potential of
reflected glare.
Photometric Data
Luminaire Efficiency describes the amount of light that is
emitted from the luminaire compared to the amount of light
that is generated by the lamps.
All fixtures absorb some of the light from the lamps. Comparing
luminaire efficiencies between fixtures allow for quick energy efficiency
ratings.
Photometric Data
Coefficient of Utilization (CU)
Coefficient of Utilization (CU) is the ratio of
lumens on the work surface to the lumens
emitted by the luminaire.
The CU is influenced by the reflectance values
of materials, size and shape of the room, and
the placement of the luminaire.
The CU is used in the lumen calculation method
that is used to determine the light levels for
ambient lighting systems.
Photometric Data
High Coefficient of Utilization (CU) values
means more light reached the work surface and
is more efficient.
Photometric Data
Coefficient of Utilization (CU) Table
pcc = reflectance value of the ceiling.
pw = reflectance value of walls
Typically the chart assumes a 20% reflectance value for the floor. (This is due
to typically dark colors and furnishings.)
Room
Ratios the
larger the
number the
larger the
room.
Photometric Data
Coefficient of Utilization (CU) Table
Note that small rooms with light colors have a high CU.
Large rooms that are dark have a low CU.
The CU represents the percentage of lumens that reach the work surface from
the fixture.
Photometric Data
Spacing Criteria describes the maximum acceptable
distance between fixtures when they are laid out in a room.
The number provided by the fixture manufacture is a ratio
that is multiplied by the distance between the work plane
that is to be illuminated and the bottom of the fixture.
Photometric Data
Spacing Criteria
If the distance between the fixture and the work plane is 6’
then the maximum spacing criteria would be 6’X1.2 = 7.2’
parallel to the fixture and 6’X1=6’ 90 degrees to the fixture.
(Remember that these are the maximum spacing
recommendations.)
7’-2” MAX
6’-0”
MAX
Photometric Data
Physical Dimensions and Finishes
Photometric Data
Lamp and Ballast Data
Photometric Data
Manufacture Photometric Data
Fluorescent with Wrap Around Prismatic Lens
Fluorescent with Prismatic Lens
Deep Cell Parabolic Fluorescent
Semi Indirect Fluorescent
Indirect Fluorescent
Compact Fluorescent Down Light
Incandescent Down Light
MR 16 Down Light
HID Down Light
Photometric Data
Manufacture Photometric Data
HID Down Light
Pendent Fixture
Wall Washer
Placement Guide
Calculation Data