Basic Concepts of Lighting Design | Electrical Engineer | MEP Design |

Basic Concepts of Lighting Design
LIGHTING EFFICIENCY | LIGHTING DESIGN | LOCAL LAW 88
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Lighting system design is a very important aspect of construction projects, and
the best performance is achieved when lighting is specified through detailed
calculations and not “rules of thumb”. However, lighting has a key difference
with other building systems like HVAC and plumbing: there is a subjective and
artistic factor involved. Lighting systems must provide adequate visibility, but
they also set the ambiance of built environments.
Effective communication is important in any engineering design process, and
this is only possible if the parties involved are familiarized with key technical
concepts. This article will provide an overview of the main terms used when
specifying lighting systems.

The Lumen: Basic Unit of Luminous
Flux
However, this leads to confusion when comparing different types of
lighting. For example, the lumen output is roughly the same for a 60W
incandescent bulb, a 15W compact fluorescent light, and a 9W LED bulb.
A common misconception about LED lighting is that you end up with a
darker room due to the reduced wattage. However, this comes from the
old practice of describing brightness with watts, when the correct unit is
the lumen.
The concept of luminous efficacy describes how efficiently a lamp
converts watts of electricity into lumens, similar to the gas mileage (MPG)
of a car. Assuming the three light bulbs described above produce 900
lumens each, the luminous efficacy values would be the following:

The Lumen: Basic Unit of Luminous
Flux
Just like a car with a higher MPG value has a lower fuel cost for a given distance
traveled, a light source with a higher lm/W value has a lower electricity cost for a given
lighting output. Upgrading to LED lighting is one of the most cost-effective energy
efficiency measures for a building.
Lumens are useful when describing the output of a lamp or fixture, but a different
measurement unit is required to describe the lighting needed by a specific area. For
example, 10,000 lumens is more than enough lighting for a small office, but the effect is
barely noticeable in a large warehouse.

Illuminance: Lumens per Unit of Area
The concept of illuminance is used to describe the lighting required for a
given occupancy, regardless of size. There are two common measurement
units:
Lux, or lumens per square meter.
Foot-candle (fc), or lumens per square foot.
1 fc = 10.7639 lux
Since illuminance is specified per unit of area, room size does not matter.
For example, an illuminance of 50 fc has the same meaning for a 500 sq.
ft. office and for a 2500 sq. ft. office, with the difference that more lighting
fixtures are required for the larger office.
In actual lighting design, the illuminance level varies due to the spacing of
fixtures and their beam shapes. However, slight variations are acceptable
as long as no areas are too dark or too bright.

Photometry: What is the Beam Shape
of Lamps and Fixtures?
Light beam shape is another important aspect considered by lighting
designers. For example, spotlights concentrate their output into a narrow
beam facing downwards, while troffers in offices spread their output over
the largest possible area to achieve uniform lighting.
Do not assume that two light bulbs can be used for the same application
just because their bases have the same shape. Using a light source with
the wrong beam shape yields poor results even if the lumen output is
calculated correctly.
The beam shape of a lamp or fixture is threedimensional,
and product models used in lighting
design software include this information for
simulation purposes. On the other hand, a 3D beam
shape cannot be represented in technical
specifications; the beam is described with
overlapping 2D figures that represent beam shape
parallel and perpendicular to the light source.

Correlated Colour Temperature and
Colour Rendering Index
Describing the colour performance of lighting requires two separate
metrics, one for the light source and another for the objects it illuminates.
The correlated colour temperature (CCT) describes the colour of the
light source itself. When dealing with the CCT, no value is considered “the
best”, since each lighting hue has different applications.
The colour rendering index (CRI) describes how faithfully the light
source renders the colours of objects and surfaces in the room. The
maximum CRI value is 100, describing a light source that matches the
quality of natural light - a higher CRI is always better regardless of the
application.

Using Temperature Values To
Describe Lighting Colour
Objects glow in a characteristic colour depending on their temperature,
and this is why lava from a volcano looks red. The same principle applies
to stars, where a yellow star like the Sun is hotter than a red star, and a
blue star is hotter than a yellow one. In physics, this behaviour is
described by an abstract concept called a “black body”, which is an object
that emits no light except when heated, and each temperature
corresponds with a specific colour glow.
Light sources are not heated to the temperature implied by their CCT
value, but it is a convenient way to assign a numerical value to their
colour. In most residential and commercial applications, the CCT value of
lighting ranges from 2700K (yellowish white) to 6500K (bluish white). In
other words, if a light product has a CCT of 4000K, it means it glows with
the same colour as a “black body” at 4000K, but the light source itself does
not reach that temperature!

Using Temperature Values To
Describe Lighting Colour
Low colour temperatures like 2700K are perceived as “warm” and they
tend to have a relaxing effect. They are preferred in areas like residential
bedrooms, hotel rooms and high-end restaurants. Warm colours are not
well suited for commercial and industrial settings, where the relaxing effect
can be counterproductive.
High colour temperatures like 6500K are perceived as “cool” and they tend
to have an energizing effect, enhancing. They are preferred in applications
where maximum visibility is required, such as high-precision
manufacturing. Cool colours may delay sleep when used in residential and
hospitality settings, and extended exposure may be described as
“stressful” by some individuals.

Effect of the Light Source on Objects
and Surfaces
Even if two light sources have the same CCT value, their lighting quality
may differ. A CRI value of 100 means the light source is a good as
sunlight.
Despite their inefficiency, incandescent and halogen bulbs offer a CRI of
100.
Fluorescent bulbs tend to have the lowest colour rendering performance,
and low-tier products can go below 70.
CRI values for LED bulbs can vary significantly depending on product
quality. Low-end products go below 70, while high-performance products
reach values close to 100.
The minimum CRI for a light bulb to get the ENERGY STAR label is 80.

Conclusion
LED upgrades have the potential to reduce lighting power consumption by
over 50 percent. If the space is air-conditioned, indirect savings are
achieved by reducing the heat footprint of lamps and fixtures. It can be
tempting to simply swap the existing lighting with the most efficient product
available, but lighting design must not be overlooked - lighting savings
should not be achieved at the expense of quality.
If your property is covered by Local Law 88, you must upgrade your
lighting to meet the NYC Energy Conservation Code by 2025. However,
you can achieve a better return on investment by exceeding the efficiency
level required, while also using the chance to improve lighting quality.