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IR
L E D
S
EL ECT ION
G
U ID E
FOR
Si114
X
P
R OX IM I T Y
A
P PLICAT IONS
1. Introduction
There are numerous factors involved in choosing the proper irLED for any given system.
This application note defines the technical requirements governing the choice of an irLED. Armed with this
information, a design engineer can choose the least expensive, most appropriate irLED suitable for the intended
application and usage. Specific irLED pricing information will not be discussed in this application note.
An important factor in an irLED choice is the industrial design of the end product. As an example, a thru-hole irLED
probably would be too tall for use inside a cell phone. Although this document occasionally touches some industrial
design topics, they are generally outside the scope of this document. From time to time, some aspects of industrial
design are mentioned, but only within the context of choosing an irLED.
There are many shapes, sizes, and footprints for LEDs that primarily are derivatives of two types, SMD and thru-
hole the standard T 1-3/4 format. In general, the following 4 categories cover typically available package types and
light emitting direction:
Standard clear or IR pass blue, thru-hole T1-3/4 format heights 5.2–8.7mm
SMT vertical emitting, heights 0.9–3.8 mm
SMT side emitting, 2–5 mm
SMT gull wing or similar, heights 2–5 mm
It is assumed that the design engineer can take the mechanical requirements implied by the industrial design and
choose the correct package type. The irLED package choice is outside the scope of this document.
This document describes the technical requirements involved in choosing an irLED for use with the Si114x active
reflectance proximity applications.
2. Terminology
In this document the following definitions apply:
Range—Distance
in cm from emitter/detector to target to be detected.
Width of coverage—The
usable width of illumination at RANGE measured perpendicular to the zero axis of
irradiation.
Radiant Flux—Total
radiant power emitted by a source expressed in (mW).
Radiant Intensity (Ie)—Equal
to the radiant flux per unit solid angle from a point light source expressed in milli-
watts per steradian (mW/sr).
Irradiance (Ee)—Power
incident on a given surface at a given distance (mW/cm
2
).
Reflectivity()—Amount
of power reflected from a surface divided by the power incident upon it expressed in (%).
Steradian (sr)—The
cone of light spreading out from the source which would illuminate one square meter of the
inner surface of a sphere of 1 m radius around the source.
NIR—The
spectrum of infrared radiation in the 720–1300 nm range.
Half-angle ()—The
angle measured with respect to the LED's light emission center line at which the radiant
intensity falls to 50% of its max value.
Ie(0)—The
peak low duty cycle pulsed radiant intensity capability of a source LED (mW/sr).
Ie(ref)—The
radiant intensity expressed in (mW/sr) of the power reflected by an object.
Ee(sensor)—The
amount of power incident to the sensor expressed in (mW/cm
2
).
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3. How to Choose an irLED
Choosing an irLED for Proximity Detection applications is analogous to purchasing a light bulb. The following steps
are involved in choosing an irLED:
1. Understand the concepts of half-angle, radiant intensity and radiant flux.
2. Determine the irLED half-angle for the intended application.
3. Determine the irradiance necessary at sensor.
4. Calculate the radiant intensity needed to arrive at the necessary irradiance.
5. Make adjustments based on the overlay.
3.1. Understanding Half-Angle, Radiant Intensity, and Radiant Flux
There are three important concepts that are linked together. Understanding these three key concepts is important
to the irLED decision.
Radiant flux is a measurement of light power. It is a power measurement expressed in watts.
The half-angle of an irLED is the angle measured with respect to the LED's light emission center line at which
the radiant intensity falls to 50% of its max value. It is an indicator of radiation pattern of the irLED.
The radiant intensity is a measurement radiant flux per unit solid angle from a point light source. Radiant
intensity is expressed in watts per steradian.
In general, the cost of an irLED is linked to the radiant flux (power).
Note:
“Radiant flux" is a power measurement and uses the measurement of watts.
To allow us to more easily relate to this concept, we can imagine making a choice between a 100-watt
incandescent bulb and a 15-watt incandescent bulb. A 100-watt bulb is generally more expensive than a 15-watt
bulb; not only in its initial cost of investment, but also in the recurring usage cost as well. The 100-watt bulb is
generally brighter than a 15-watt bulb also.
To minimize cost, the challenge is to choose the right irLED that emits the right amount of illumination (irradiation)
depending on the size and distance of the target we need to illuminate (irradiate).
The next concept is “half-angle.” The half-angle of an irLED describes the radiation pattern of an irLED. The radiant
intensity at the half-angle is ½ the radiant intensity compared to the axial direction. Refer to Figure 1.
The irLED does not radiate equally in all directions. An irLED with a narrow half-angle concentrates most of its
power at a smaller region of space. The radiant intensity becomes higher as a result.
Conceptually, imagine taking a 15-watt bulb and placing it in a parabolic reflector. Most of the light power is
redirected to a certain direction. Fundamentally, the radiant flux (power) has not changed, but, in the intended
direction, the radiant intensity increases significantly. The higher radiant intensity is achieved by redirecting the
power towards the intended illumination target.
Given an irLED with a similar half-angle radiation pattern, the irLED with higher radiant intensity in the axial
direction can only do so if the radiant flux is higher. In the same way, given an irLED with the same radiant flux
(power), an irLED with a larger half-angle would have a lower radiant intensity in the axial direction.
These three concepts of half-angle, radiant intensity and radiant flux are all linked together. It is good engineering
practice to direct all of the available light only in the direction which will illuminate the intended target. This way, it is
possible to choose the least expensive irLED which can do the intended job.
In general:
Cost is proportional to Radiant Flux (W)
Radiant Flux (W) is proportional to Half Angle x Radiant Intensity.
What these equations imply is that given a fixed cost (implied by the radiant flux rating of the irLED), there is a
trade-off between the radiation pattern (half-angle) and the radiant intensity of the irLED.
We can use a common reflector and a flashlight to allow us to relate to the equations stated above.
A flashlight consists of an incandescent bulb. Without the reflector, a light bulb radiates in almost all directions
(except the direction where the bulb connects to the socket). The reflectors of the flashlight direct the light that
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otherwise would have been wasted. The radiant intensity increases in a given direction, while radiant intensity
becomes absent in other directions.
Given that the concepts of half-angle, radiant intensity, and radiant flux are related, it is sufficient to describe the
requirements for two of these concepts. It is best to examine the half-angle requirements and the radiant intensity.
For effective cost minimization, it is a matter of choosing the irLED with the minimum half-angle radiation pattern
that meets the radiant intensity requirements for the application.
One final detail is that much of the discussion on radiant power and radiant intensity thus far has been mentioned
as a property of the irLED. This is not strictly true. Making such a statement would be similar to saying that a 100-
watt bulb expends 100-watts of power when it is not in a light socket.
The reality is that for any given irLED,
Radiant Flux = k x irLED current
Radiant Intensity = k x irLED current
When choosing an irLED, the radiant flux and radiant intensity are specified given a fixed current.
In a similar way, a 100-watt bulb does not always expend 100 watts. The purchase of a 100-watt bulb presupposes
that it is to be used in a known power system. For example, in the United States, we should expect that a 100-watt
bulb would dissipate 100 watts only when it is connected to a 120 Vrms power system.
It is possible to take that same bulb and use it in France where the power system is 220 Vrms. For a short period of
time before the filament burns out, the 100-watt bulb would actually be emitting light based on 484 watts of power
and would be much brighter for this short period of time.
The radiant intensity and radiant flux rating of any irLEDs are typically associated with a given DC current level. It is
generally permitted to drive the irLEDs with a higher peak current as long as the duty cycle is reasonably short.
Pulsing the irLED allows higher radiant intensity during the on-period of the pulse.
3.2. Calculating the irLED Half-Angle
Distance and width of coverage are used to determine the half-angle () of the irLED to be selected. Once you
have decided on the range required for your application it is a simple matter of trigonometry to determine the half-
angle as shown in Figure 1. For detailed information related to half-angle and LED power definitions see the
Appendix (Optical Power Primer).
Figure 1. Width of Coverage
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Example: If the distance to the target is 100 cm and width of coverage 100 cm, then:
=
arctan
100
0.5
100
=
26.6 degrees
irLEDs come in standard half-angles of 5, 10, 15, 20, 30, 60, etc. Choose the nearest half-angle available; in this
case an angle of 30 degrees would be appropriate to fully cover the target. An angle of 20 degrees could also work
if the designer trades off a slightly longer distance to obtain the same width of coverage (e.g. D= (100 * 0.5) ÷
tan(20) = 137cm).
3.3. Deciding Target Irradiance at the Sensor
In prior sections it was stated that an irLED decision is contingent upon knowing the half-angle and the radiant
intensity. The method of choosing the irLED radiant intensity is covered in the next section. However, before it is
possible to derive the radiant intensity of the irLED, it is important to know the minimum irradiance level needed at
the sensor. This section provides some guidelines for choosing the proper irradiance target.
Ee(sensor) is the power in mW/cm
2
incident to the surface of the Si114x detector assuming that the irLED emitter
is beside the Si114x, and that the target object is directly above both the irLED and the Si114x.
Fundamentally, the Si114x must be able to measure an increase in irradiance due to the irLED, compared to the
background ambient level. The Si114x internally makes two measurements. The first measurement is made with
the irLED disabled to estimate the background IR radiation. The second measurement is with the irLED enabled.
The Si114x reports the difference between these two measurements. The difference in reading is proportional to
the reflected light from the target object, illuminated by the irLED.
The required irradiance at the sensor is a function of the ambient IR noise level, the IR ambient level, the light
source type, and the ADC setting used by the Si114x.
Assuming that the sensor is operating indoors, illuminated with artificial light sources with at maximum expected
light levels of less than 450 lux incandescent or 1 klx of CFL lighting, the default Si114x Proximity ADC Setting can
be used. Under this condition, 4 uW/cm
2
is the minimum irradiance target at the sensor.
For applications that need to operate under high levels of artificial lighting (but less than 6.5 klx incandescent or
14.5 klx of CFL lighting), the HSIG bit in the PS_ADC_MISC needs to be set. The minimum irradiance at the
sensor in this case is 58 uW/cm
2
.
For applications operating outdoors but not under direct sunlight, the PS_ADC_MISC HSIG bit should be set. The
minimum irradiance target of 7 uW/cm
2
would be sufficient.
For use under direct sunlight, in addition to setting the HSIG bit, the PS_ADC_MUX may also need to be
configured to use the smaller IR photodiode to avoid saturation. In this case, the minimum irradiance target is 45
uW/cm
2
.
Table 1. Typical Ee Design Targets
Ee (sensor)
Minimum
0.25 uW/cm
2
0.5 uW/cm
2
1 uW/cm
2
2 uW/cm
2
4 uW/cm
2
8 uW/cm
2
HSIG Bit
IR Photo
Diode
Large
Large
Large
Large
Large
Large
ADC
Integration
Time
408.6
s
204.8
s
102.4
s
51.2
s
25.6
s
25.6
s
Usage (no overlay assumed)
No
No
No
No
No
Yes
Artificial Lighting CFL < 63 lx; Incandescent < 23 lx
Artificial Lighting CFL < 125 lx; Incandescent < 56 lx
Artificial Lighting CFL < 250 lx; Incandescent < 112 lx
Artificial Lighting CFL < 500 lx; Incandescent < 225 lx
Artificial Lighting CFL < 1 klx; Incandescent < 450 lx
Artificial Lighting CFL < 2 klx; Incandescent < 900 lx
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Table 1. Typical Ee Design Targets (Continued)
Ee (sensor)
Minimum
16 uW/cm
2
32 uW/cm
2
58 uW/cm
2
7 uW/cm
2
22 uW/cm
2
45 uW/cm
2
HSIG Bit
IR Photo
Diode
Large
Large
Large
Large
Small
Small
ADC
Integration
Time
25.6
s
25.6
s
25.6
s
25.6
s
51.2
s
25.6
s
Usage (no overlay assumed)
Yes
Yes
Yes
Yes
Yes
Yes
Artificial Lighting CFL < 4 klx; Incandescent < 1.8 klx
Artificial Lighting CFL < 8 klx; Incandescent < 3.6 klx
Artificial Lighting CFL < 14.5 klx; Incandescent < 6.5 klx
Indirect Sunlight < 16 klx
Direct Sunlight < 80 klx
Direct Sunlight < 190klx
In general, for most hand-held applications, 8 uW/cm
2
is a good minimum irradiance target to use for choosing an
irLED. This allows operation under typical indoor lighting conditions and allows operation outdoors. For the rest of
this document, 8 uW/cm
2
is assumed in all calculations. Choosing a higher Ee value at the sensor would lead to an
irLED with a higher radiant intensity, leading to trade-offs on radiation pattern or irLED cost.
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