When reviewing outdoor LED floodlight specifications, it is important to first confirm which luminous intensity threshold is used to define the beam angle.
In the Technical Requirements for LED Nightscape Lighting Applications, the luminous efficacy table for white LED floodlights defines B10% at 10% of the maximum luminous intensity, while B50% is calculated at 50% of the maximum luminous intensity.
These two measurements represent different angles and should not be confused with each other.
The unit of luminous efficacy is lumens per watt (lm/W).
Understanding the Difference Between Half-Intensity Beam Angle and 10% Intensity Angle
The term beam angle commonly refers to the half-intensity beam angle, which is the angle between the two directions where luminous intensity drops to half of its maximum value on the light distribution curve. B50% uses this measurement method.
B10%, on the other hand, is called the 10% intensity angle, or one-tenth intensity angle.
In English lighting terminology, B10% is commonly referred to as the Field Angle. It is often called the full angle in Chinese technical documentation.
This measurement is commonly found in North American IES standards and frequently appears in outdoor LED floodlight specification sheets from Chinese manufacturers. The technical requirements mentioned above also use this parameter.
Understanding these differences is helpful when comparing floodlights or reviewing photometric data as part of a solar street lighting design and calculation.
Reading Both Intensity Thresholds from a Photometric Distribution Curve
Consider a photometric distribution curve with a maximum luminous intensity of 12,889 cd.
To determine the half-intensity beam angle, divide the luminous intensity scale into ten equal parts. The 50% intensity level corresponds to the fifth division circle.
Find the two points where this circle intersects the luminous intensity distribution curve. Then draw a line from each intersection point to the origin.
The angle between these two lines is the half-intensity beam angle.
In the example, the manually measured angle is approximately 29°.
Half-intensity beam angle at 50% of maximum luminous intensity.
To determine the 10% intensity angle, follow the same procedure, but use the first division circle instead of the fifth.
Connect the two intersection points between this circle and the luminous intensity curve to the origin.
The resulting angle is approximately 47°.
Field angle at 10% of maximum luminous intensity.
Checking the Measurements Using an IES File
Photometric curves in printed catalogs are often relatively small, making it difficult to read angles accurately.
If the manufacturer provides an IES file, it can be opened using compatible photometric software to examine both the distribution curve and its technical parameters.
Figure 3 shows the same photometric distribution displayed in software, while Figure 4 shows the parameter panel listing both the 50% and 10% intensity angles.
Figure 3. Photometric distribution curve displayed in lighting software.
Figure 4. Beam angle and field angle parameters.
The software reports a 50% beam angle of 29.2° and a 10% field angle of 47.6°.
These results closely match the manually measured values of 29° and 47°.
When checking beam angles, always confirm both the angle value and the corresponding percentage of maximum luminous intensity.
This is particularly useful when comparing manufacturer specifications or evaluating the light distribution requirements for outdoor lighting installations. Related considerations, including mounting height and spacing, are discussed in our guide to solar street light pole height and distance.
For procurement teams and installers, requesting the original photometric data is also a useful part of evaluating international standards for high-quality solar street lights.
Frequently Asked Questions
What is the difference between B50% and B10% in LED floodlights?
B50% is the angle measured where luminous intensity falls to 50% of its maximum value, while B10% is measured at 10%. B10% generally represents a wider angle than B50%.
What does half-intensity beam angle mean?
The half-intensity beam angle is the angle between two directions where the luminous intensity reaches 50% of the maximum intensity. It is commonly represented as B50%.
What is the field angle of an LED floodlight?
The field angle is generally measured at 10% of the maximum luminous intensity. It is also called the 10% intensity angle or B10%.
Why is the B10% angle larger than the B50% angle?
For a typical single-peaked floodlight distribution, luminous intensity gradually decreases away from the center of the beam. The 10% intensity points are farther from the center than the 50% points, resulting in a wider angle.
How can I measure the beam angle from a photometric curve?
Find the points where the luminous intensity curve intersects the chosen intensity threshold, such as 50% or 10% of the maximum value. Connect those points to the origin and measure the angle between the lines.
Can I verify beam angle values using an IES file?
Yes. Compatible photometric software can display the luminous intensity distribution and provide beam angle and field angle measurements based on the IES data.
Why should buyers check whether a beam angle is B50% or B10%?
Because the same floodlight can have different B50% and B10% values. Confirming the measurement threshold helps buyers compare product specifications correctly and avoid confusing the main beam width with the wider field angle.
