INDEX
1. Why SAN-EI Provides Angular Information
Solar simulator performance is generally evaluated by the following key items:
- Spectral irradiance distribution
- Irradiance level
- Spatial non-uniformity over the effective irradiation area
- Temporal instability
These are essential parameters for evaluating the quantity, distribution, and stability of simulated sunlight.
However, in some applications, the angle at which light enters the sample surface may also affect the test result.
This is especially important in applications such as:
- Fine pattern exposure
- Photoresist evaluation
- Optical material evaluation
- Evaluation of samples with angular dependence
- Thin films, multilayer films, and anti-reflection coatings
- Small-area devices requiring well-defined irradiation conditions
For this reason, SAN-EI provides angular information when it is useful for understanding the irradiation conditions more accurately.
Angular characteristics do not replace the basic performance items of a solar simulator.
They are additional technical information that helps users understand the nature of the light reaching the sample surface.
SAN-EI’s purpose is not to make the equipment appear better by adding more specifications.
Our purpose is to help users judge whether the irradiation conditions are suitable for their specific test purpose.
Therefore, in addition to spectral characteristics, spatial non-uniformity, and temporal instability, SAN-EI also provides angular information when it is relevant to the application.
2. SAN-EI’s Basic Policy
SAN-EI regards angular information as supplementary information for correct use and correct evaluation.
Angular values should not be shown as isolated numbers.
The meaning of an angular specification depends on the definition, measurement position, measurement range, and calculation method.
For this reason, SAN-EI provides angular specifications together with the following information whenever possible:
- What the value represents
- Where the value was measured
- Which part of the light was evaluated
- How the value was calculated
- For what purpose the value is useful
This approach helps users understand and compare angular characteristics without misunderstanding.
3. Transition from Conventional CHA and DA Indications
SAN-EI has previously provided angular values such as CHA (Collimation Half Angle) and DA (Declination Angle) in response to user requests.
These indications have been used mainly to satisfy specific user specifications or application requirements.
However, angular terms such as CHA and DA may be interpreted differently depending on the field, application, or user.
Even when the same term is used, the meaning of the value may change depending on the measurement method, measurement plane, evaluation range, and calculation method.
Therefore, SAN-EI is moving toward a clearer method of displaying angular specifications.
In the new approach, angular values will be shown together with their definitions, measurement conditions, and calculation methods.
This does not mean that SAN-EI will stop supporting conventional CHA and DA indications.
When users require CHA or DA, SAN-EI will continue to provide them as necessary.
However, SAN-EI will provide these values with clear definitions and conditions, rather than showing the numbers alone.
Our intention is not to reject the conventional indications, but to make angular information more accurate, more reproducible, and easier to understand.
4. Main Angular Specifications
4.1 Incident Angle
The incident angle is the angle at which light enters the sample surface.
Normally, the direction perpendicular to the sample surface is defined as 0°.
The incident angle shows how far the incoming light is inclined from this perpendicular direction.
The incident angle may affect reflection, absorption, transmission, and interference at the sample surface.
This is especially important for thin films, multilayer films, optical coatings, solar cells, and photoresists.
For this reason, incident angle information can be useful for confirming the reproducibility and comparability of test results.
4.2 Optical Axis Angle
The optical axis angle indicates how much the central direction of the irradiation light is tilted from the normal direction of the sample surface.
Ideally, the central direction of the light should be perpendicular to the sample surface.
In actual optical systems, however, the optical axis may be slightly tilted due to the lamp, reflector, lens, filter, working distance, or mechanical alignment.
Displaying the optical axis angle helps users understand how well the central direction of the irradiation light is aligned with the sample plane.
This information is especially important for precise positioning, fine pattern exposure, and evaluation of samples with angular dependence.
4.3 Divergence Angle
The divergence angle indicates how widely the irradiation light spreads in angle when it reaches the sample surface.
The light from a solar simulator is not perfectly parallel.
Because of the lamp source, reflector, lens system, filter, and working distance, the light reaches the sample surface with a certain angular spread.
The divergence angle is used to describe this angular spread.
A smaller divergence angle means that the light is closer to parallel light.
A larger divergence angle means that light enters the sample from a wider range of directions.
However, a smaller divergence angle is not always better.
For applications requiring a wide and uniform irradiation area, a certain angular spread may be necessary.
For fine pattern exposure or samples with strong angular dependence, a smaller angular spread may be more important.
Therefore, the divergence angle should not be used as a simple ranking value.
It should be evaluated in relation to the intended application.
4.4 Half-Angle and Full Width at Half Maximum
The half-angle is the angle at which the angular intensity falls to 50% of the maximum intensity.
It is used to understand how concentrated the main part of the irradiation light is around the central direction.
In some cases, the value is shown as a half-angle from the center.
In other cases, it is shown as the full width at half maximum.
For example:
- Half-angle: ±2.5°
- Full width at half maximum: 5.0°
These may describe the same angular distribution, but the expression is different.
Therefore, when comparing values, it is important to confirm whether the value is shown as a half-angle or as a full width.
4.5 CHA (Collimation Half Angle)
CHA is one of the angular indications that SAN-EI has provided in response to user requests.
It has been used to express angular characteristics in a simple form for specific applications.
However, the definition of CHA may vary depending on the user or technical field.
Therefore, when SAN-EI provides CHA, we will also clarify the definition, measurement position, evaluation range, and calculation method.
CHA will continue to be supported when it is required by users or existing specifications.
At the same time, SAN-EI will not rely only on CHA.
When necessary, CHA will be explained together with angular distribution, optical axis angle, divergence angle, or other defined angular information.
4.6 DA (Declination Angle)
DA is also one of the angular indications that SAN-EI has provided according to user requirements.
DA is used to describe the angular deviation of the irradiation light from a defined reference direction, such as the optical axis or the normal direction of the sample plane.
However, as with CHA, the meaning of DA may vary depending on the definition and measurement method.
Therefore, when SAN-EI provides DA, we will clarify the measurement position, measurement plane, angular range, and calculation method.
DA will continue to be supported for compatibility with conventional user requirements.
At the same time, SAN-EI will gradually move toward clearer and more reproducible angular specifications.
5. Measurement Method
5.1 Measurement Position
Angular characteristics are measured at the sample plane, normally at the specified working distance.
The working distance is the position where the sample is placed and where the optical performance is evaluated.
If the working distance changes, the irradiation area, irradiance, spatial non-uniformity, and angular distribution may also change.
Therefore, when angular specifications are shown, the measurement position must be clearly stated.
Typical information includes:
- Measurement position
- Working distance
- Measurement plane
- Effective irradiation area
- Measurement point or points
- Reference direction for angle measurement
5.2 Measurement Target
The measurement target is the angular distribution of the irradiation light at the sample plane.
Depending on the purpose, the measurement may include:
- Angular distribution at the center of the effective irradiation area
- Angular distribution at multiple points within the effective irradiation area
- Tilt of the optical axis
- Angular spread of the light
- Ratio of light included within a specified angular range
A single measurement point may not fully represent the angular characteristics over the entire effective irradiation area.
For high-precision applications, measurements at multiple points may be required.
5.3 Measurement Equipment
Angular characteristics may be measured using suitable instruments or measurement systems, such as:
- Angular distribution measurement equipment
- Goniometer-based measurement system
- A slit and sensor measurement system
- Camera-based optical axis confirmation system
- Dedicated fixtures for incident angle measurement
The type of measurement equipment affects the angular resolution, measurable range, and interpretation of the result.
Therefore, SAN-EI records the measurement equipment and conditions when angular specifications are provided.
5.4 Measurement Conditions
The following conditions should be recorded when angular characteristics are measured:
- Model name
- Lamp type
- Lamp current or operating condition
- Filter configuration
- Working distance
- Measurement plane
- Effective irradiation area
- Measurement point
- Measurement equipment
- Angular measurement range
- Angular resolution
- Measurement environment
- Measurement date
By recording these conditions, the reproducibility and comparability of angular information can be improved.
6. Calculation Method
6.1 Optical Axis Angle
The optical axis angle is calculated by determining the central direction of the measured angular distribution.
The angle between this central direction and the normal direction of the sample surface is then shown as the optical axis angle.
Example:
Optical axis angle: 0.8°
This means that the central direction of the irradiation light is tilted by 0.8° from the normal direction of the sample surface.
6.2 Divergence Angle
The divergence angle is calculated from the angular spread of the measured light distribution. Several methods may be used depending on the purpose, such as:
- The angular range where the intensity is 50% or more of the maximum intensity
- The angular range that includes a specified percentage of the total light
- The angular range where the intensity exceeds a specified threshold
When SAN-EI shows a divergence angle, the calculation method will be stated.
Example:
Divergence angle: ±3.0°
Condition: angular range where the intensity is 50% or more of the maximum intensity
This means that the main part of the light distribution is within ±3.0° from the central direction under the stated condition.
6.3 Half-Angle
The half-angle is calculated by finding the angle where the angular intensity becomes 50% of the maximum intensity.
SAN-EI will clarify whether the value is shown as a half-angle or as a full width.
Example:
Half-angle: ±2.5°
or
Full width at half maximum: 5.0°
The numerical meaning is different depending on the expression.
Therefore, the definition must be confirmed before comparing values.
6.4 CHA (Collimation Half Angle) and DA (Declination Angle)
When CHA or DA is provided, the calculation method will follow the user-specified definition or the conventional specification requested by the user.
However, SAN-EI will also clarify the following information:
- Definition of CHA or DA
- Measurement position
- Measurement range
- Evaluation angle range
- Calculation method
- Whether the value is shown as a half-angle or full width
This allows SAN-EI to maintain compatibility with conventional user requirements while avoiding misunderstanding.
7. Important Notes
Angular characteristics depend on the measurement conditions.
Values measured with different working distances, different irradiation areas, different measurement methods, or different definitions cannot be compared directly.
A smaller angular value is not always better.
In some applications, a certain angular spread may be necessary to obtain a uniform irradiation area.
In other applications, such as fine pattern exposure or evaluation of angular-dependent samples, a smaller angular spread may be preferred.
Therefore, angular specifications should not be used as simple ranking values.
They should be used as technical information for judging whether the irradiation condition is suitable for the intended application.
SAN-EI provides angular information to support correct understanding, correct use, and correct evaluation of solar simulators.
8. SAN-EI’s Position
SAN-EI will provide angular specifications as supplementary technical information when they are useful for the application.
We will continue to support conventional CHA and DA indications when users require them.
At the same time, we will gradually move toward clearer angular specifications with defined measurement conditions and calculation methods.
Our goal is to provide information that helps users understand the actual irradiation conditions at the sample plane.
SAN-EI believes that solar simulator performance should not be explained only by isolated numerical values.
It should be explained together with the measurement conditions, definitions, and purpose of use.
Angular characteristics are one part of this approach.