Luminous
intensity
How much brighter is a flashlight (torch) than a candle? Clearly it depends on how powerful the
flashlight is, but there are other factors too. For example, the candle spreads its light in all
directions whereas the flashlight is highly directional. This makes direct comparisons a little
tricky, but not impossible. This is where the SI quantity of luminous intensity is useful. It has
the SI unit candela and the symbol cd.
How many candles are as bright as a flashlight?
The next SI Unit is the kilogram (kg). One of the extraordinary things about the kilogram is that
until 2019 it was a real object, hidden in a nuclear bomb-proof bunker outside Paris. Other SI
units are available from the menus at the top of the page.
For much of history the candle was used as a measure of the intensity of light. This made
sense in a world without light bulbs or other forms of artificial light, even if each country
tended to use a slightly different kind of candle as its "standard". Things started to get
complicated not just with the invention of light bulbs, but also with a greater understanding of
the different colors of light. For example, the Sun's light is composed of all of the different
colors mixed together, but, as seen from space, is faintly yellow in color, meaning that it
shines slightly more brightly in yellow than the other color components.
For these and other reasons a better definition of luminous intensity was needed. The
modern SI quantity of luminous intensity takes into account the color of the light and its
direction. It is based on a frequency (light color) of 540 x 10
12
cycles per second (hertz),
which is roughly yellowish-green and is a color that the human eye is highly sensitive to.
Now the surprisingly easy bit. One candela is almost exactly equivalent to one candlepower (I
did say it was easy!). So why is comparing a candle and flashlight tricky? To answer this we
need to consider how the light spreads out from its source as well as taking into account a
derived quantity called lumens.
Candlepower and candela are both measurements of light at source, but neither tells us how
powerful the light is some distance away from the source. Instead, we measure the amount of
light illuminating a surface area, which is called, naturally enough, the illuminance. The result
is measured in lumens, with 1 lumen = 1 candela x steradian. For our purposes here we can
think of the the latter term as an area, as the following example illustrates:
Imagine a transparent 1 metre radius sphere surrounding a candle. Its surface area will be
given by 4 pi r
2
, so the surface area of our sphere is:
Seen from space the Sun is yellow
4 pi 1
2
= 12.57 m
2
The amount of energy passing through 1 square metre of the transparent sphere is 1 lumen,
and so it follows that 1 candlepower is 12.57 lumens.
Many bulbs are now rated by the amount of illumination given at a distance. In SI units the
standard is 1 lumen per square metre and called 1 lux.
Now let's return to flashlights, most of which are designed to be highly directional. If we have
a 1 candela (i.e. 1 candlepower) flashlight which just happens to have a beam that
illuminates 1 square metre from 1 metre distance, then that square metre will have an
illumination of 12.57 lumens, as compared with 1 lumen for the candle. This is why it's not
straightforward to compare the output of a flashlight with a candle. In short, when we want to
measure how powerful a light source is we need to consider its nature or purpose - a half-
candlepower flashlight will be more useful than a candle for finding something in the distance
at night. In this case the candle has more candlepower (i.e. a high candela rating) at its
source, but the flashlight's light is focused and so more useful.
Finally, to put things into perspective, it's worth noting that a full moon on a clear night results
in the Earth being illuminated by about a quarter of 1 lux at the Earth's surface. In
comparison a typical living room will have an illumination of about 50 lux and direct sunlight
results in anything up to about 130,000 lux at the Earth's surface.
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Luminous intensity
How much brighter is a flashlight (torch) than a candle? Clearly it
depends on how powerful the flashlight is, but there are other
factors too. For example, the candle spreads its light in all
directions whereas the flashlight is highly directional. This makes
direct comparisons a little tricky, but not impossible. This is
where the SI quantity of luminous intensity is useful. It has the
SI unit candela and the symbol cd.
How many candles are as bright as a flashlight?
The next SI Unit is the kilogram (kg). One of the extraordinary
things about the kilogram is that until 2019 it was a real object,
hidden in a nuclear bomb-proof bunker outside Paris. Other SI
units are available from the menus at the top of the page.
For much of history the candle was used as a measure of the
intensity of light. This made sense in a world without light bulbs
or other forms of artificial light, even if each country tended to
use a slightly different kind of candle as its "standard". Things
started to get complicated not just with the invention of light
bulbs, but also with a greater understanding of the different
colors of light. For example, the Sun's light is composed of all of
the different colors mixed together, but, as seen from space, is
faintly yellow in color, meaning that it shines slightly more
brightly in yellow than the other color components.
For these and other reasons a better definition of luminous
intensity was needed. The modern SI quantity of luminous
intensity takes into account the color of the light and its
direction. It is based on a frequency (light color) of 540 x 10
12
cycles per second (hertz), which is roughly yellowish-green and
is a color that the human eye is highly sensitive to.
Now the surprisingly easy bit. One candela is almost exactly
equivalent to one candlepower (I did say it was easy!). So why
is comparing a candle and flashlight tricky? To answer this we
need to consider how the light spreads out from its source as
well as taking into account a derived quantity called lumens.
Candlepower and candela are both measurements of light at
source, but neither tells us how powerful the light is some
distance away from the source. Instead, we measure the amount
of light illuminating a surface area, which is called, naturally
enough, the illuminance. The result is measured in lumens, with
1 lumen = 1 candela x steradian. For our purposes here we can
think of the the latter term as an area, as the following example
illustrates:
Imagine a transparent 1 metre radius sphere surrounding a
candle. Its surface area will be given by 4 pi r
2
, so the surface
area of our sphere is:
Seen from space the Sun is yellow
4 pi 1
2
= 12.57 m
2
The amount of energy passing through 1 square metre of the
transparent sphere is 1 lumen, and so it follows that 1
candlepower is 12.57 lumens.
Many bulbs are now rated by the amount of illumination given at
a distance. In SI units the standard is 1 lumen per square metre
and called 1 lux.
Now let's return to flashlights, most of which are designed to be
highly directional. If we have a 1 candela (i.e. 1 candlepower)
flashlight which just happens to have a beam that illuminates 1
square metre from 1 metre distance, then that square metre will
have an illumination of 12.57 lumens, as compared with 1 lumen
for the candle. This is why it's not straightforward to compare the
output of a flashlight with a candle. In short, when we want to
measure how powerful a light source is we need to consider its
nature or purpose - a half-candlepower flashlight will be more
useful than a candle for finding something in the distance at
night. In this case the candle has more candlepower (i.e. a high
candela rating) at its source, but the flashlight's light is focused
and so more useful.
Finally, to put things into perspective, it's worth noting that a full
moon on a clear night results in the Earth being illuminated by
about a quarter of 1 lux at the Earth's surface. In comparison a
typical living room will have an illumination of about 50 lux and
direct sunlight results in anything up to about 130,000 lux at the
Earth's surface.
