STSF2833 - LAB REPORT
MUHAMMAD AMEERUL AIFFIX BIN GHAZALI (A174049)
DETECTION OF LIGHT INTENSITY FROM VARIOUS LIGHT SOURCES
OBJECTIVE
i.
ii.
To determine the relationship between the distance from the light source and the intensity of
the light.
To prove if the relationship that exists can be applied to all radiating light sources.
INTRODUCTION
This experiment is done to determine the relationship between the light intensity and distance
and also if this relationship exists, does it apply for all radiating light source. Luminous intensity is a
measure of the wavelength-weighted power emitted by a light source in a particular direction per unit
solid angle, based on the luminosity function, a standardized model of the sensitivity of the human
eye. The SI unit of luminous intensity is the candela (cd). The irradiance expresses radiation power
received by the unit area of the illuminated surface. The unit of irradiance is Wm -2.
The irradiance of a surface due to the point of linear light source depends on the orientation
of the surface with respect to the source, radiant intensity, and the distance to the surface and the
formula is;
𝐸=
𝑝ℎ𝑜𝑡𝑜𝑚𝑒𝑡𝑟𝑖𝑐 𝑓𝑙𝑢𝑥, Φ
𝐼
= 2
𝑎𝑟𝑒𝑎, 𝐴
𝑟
Where E is the illuminance, I is power (flux) per unit solid angle and r is distance
Point sources are defined as a single point in space that produce energy (in this case,
illumination) equally in all direction. In a point light source, luminous intensity is dependant on r -2,
meanings doubling the r will result in the E dropping to one fourth of its value. The intensity of the
influence at any given radius, r is the source strength divided by the area of the surface at that radius.
EXPERIMENT SET-UP
FIGURE 1
PROCEDURE
1) The apparatus was set-up as shown in FIGURE 1. The Lux Meter in the smartphone was used
with the settings on Lux.
2) The back camera on the smartphone was set so as to it faces the bulb.
3) The measurement for illumination was recorded by using the Lux Light Meter application at a
distance of 10 cm away from the light source (torchlight). Three readings were taken in order
to find the average reading of luminosity.
4) Step 3 was repeated by moving the smartphone away from the light source at intervals of 5
cm up till 80 cm and the subsequent measurements of luminosity were recorded.
5) Steps 1 to 4 were then repeated by using the secondary light source (bed lamp) to obtain
readings of luminous intensity.
6) Graph of illumination against distance was plotted by using the obtained data.
DATA AND RESULTS
Light source 1: Torchlight
Distance (cm)
Illuminance (Lux)
2
3
195
195
121
122
72
73
49
51
37
38
30
27
25
23
20
18
16
15
12
11
11
10
9
10
10
9
9
9
8
9
TABLE 1
1
194
123
73
52
36
30
24
19
15
11
10
10
9
8
8
10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
Average illuminance (Lux)
194.67
122.00
73.67
50.67
37.00
29.00
24.00
19.00
15.33
11.33
10.33
9.67
9.33
8.67
8.33
Uncertainty of measured luminosity = standard deviation at the 10 cm reading
(194 − 194.67)2 + (195 − 194.67)2 + (195 − 194.67)2
=√
3−1
= 0.577
= ±0.6
Graph Illuminance against Distance (1)
250
Illuminance (Lux)
200
150
100
50
0
0
10
20
30
40
50
Distance (cm)
GRAPH 1
60
70
80
90
Light source 2: Bed lamp
Distance (cm)
1
111
79
55
41
34
30
26
24
21
21
20
19
19
18
19
10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
Illuminance (Lux)
2
112
76
54
42
34
28
25
21
22
21
21
20
19
20
20
TABLE 2
Average illuminance (Lux)
3
112
79
53
42
34
29
25
21
21
20
20
20
20
19
18
111.67
78.00
54.00
41.67
34.00
29.00
25.33
22.00
21.33
20.67
20.33
19.67
19.33
19.00
19.00
Uncertainty o measured luminosity = standard deviation at the 10 cm reading
(111 − 111.67)2 + (112 − 111.67)2 + (112 − 111.67)2
=√
3−1
= 0.577
= ±0.6
Graph Illuminance against Distance (2)
120
Illuminace (lux)
100
80
60
40
20
0
0
10
20
30
40
50
Distance (cm)
GRAPH 2
60
70
80
90
DISCUSSION
The experiment was carried out by facing the back of the smartphone towards the source of
the light and the illuminance readings were expressed on the smartphone. Based on the obtained
data, the graph of illuminance against distance was plotted for both of the light source. Based on the
graphs that was plotted, the results of this experiment have successfully shown the relationship
between illumination of light and the distance from a point source. The obtained relationship is seen
to be logarithmic. This is same with the theory that states that the illumination of light is proportional
to r2. The obtained graphs show and prove the inverse square relationship that exist between the
illumination of the light source and the distance from the source. So, according to this law, when the
distance from the light source is doubled, the illumination is a one fourth of its original value and so
on.
Apart from the relationship between illumination and the distance from light source, the
purpose of this experiment was also to find out whether the relationship was valid for all types and
shapes of radiating light source. From this experiment, we know that light from a spherical source will
drop in an inverse square as the illumination is equally distributed in all directions from the source.
However, for planar wavefronts sources, the illumination does not fall although it does not go long
distances due to practical limitations. For cylindrical sources, the illumination drops in a one over
distance. Thus, it than be concluded that the inverse law does not valid for all sources that are not
considered point light sources and it is dependent on wavefront shape as well.
ERRORS
PRECAUTIONS
There is light from the environment that The experiment should be conducted in a dark
interferes the accuracy of the reading.
room or at night to minimise foreign light
pollution.
The precision and sensitivity of the light meter Everyone needs to use the same and reliable
sensor on the smartphone is dependent on the application to get the truly accurate value at
type of application used as well as the type of least.
camera on the smartphone. This will cause the
data to not be consistent for everyone.
There is no proper tool to maintain the Multiple measurements were taken at each
orientation of the smartphone. So, its distance interval to find the average values that
orientation will change each time it is moved. is more reliable.
This will lead to inaccurate data.
CONCLUSION
Based on the obtained data and the plotted graph, the first objective has been answered i.e.,
the relationship between the illuminance and the distance of the light source which is logarithmic. It
is verified that the illuminance of point light sources obeys the inverse square law of propagation. For
the second objective, we can conclude that the relationship between illuminance and distance from
source is also dependent on the geometry or the wavefront shape of the source. In conclusion, the
purpose of this experiment has been successfully achieved.
REFERENCES

Energy and Intensity of Light (2019). Retrieved on:
https://phys.libretexts.org/Courses/University_of_California_Davis/UCD%3A_Physics_7C__General_Physics/10%3A_Electromagnetism/10.4%3A_Electromagnetic_Waves%3A_Light/3
._Energy_and_Intensity_of_Light

How to Measure Light Intensity (2020). Retrieved on: https://bioslighting.com/humancentric-lighting/how-to-measure-light-intensity/

More on Brightness as a Function of Distance (2020). Retrieved
https://imagine.gsfc.nasa.gov/features/yba/M31_velocity/lightcurve/more.html
on: