How to Measure Light Correctly?

For lighting designers using spectrometers (e.g., devices like the In. Licht Ultra), measurement distance impacts accuracy — and determines whether you’re capturing the light source itself, ambient/task lighting, or human-centric lighting.

What Are You Measuring?

1) Direct Light (Source Measurement)

Key metrics to measure include:

🔹 Correlated Color Temperature (CCT)

🔹 Spectral Power Distribution (SPD) — including dominant wavelength (λd), peak wavelength (λp), and peak intensity (λpv)

🔹 Color Rendering Metrics — CRI (Ra and Re), and TM-30 (Rf and Rg)

🔹 Flicker — flicker frequency, flicker percentage, flicker index, and Stroboscopic Visibility Measure (SVM)

🔹 SDCM (MacAdam Ellipses)

🔹 Chromaticity Coordinates (x, y) and blackbody deviation (Duv)

🔹 Blue Light Hazard Ranking (RG)

Room Condition Requirements

1. Dark or Controlled Lighting Environment

🔹 Ideally, perform measurements in a dark room or one with minimal ambient light to avoid stray light interference.

🔹 If not possible, use black matte surfaces around the test area to reduce reflections.

2. Avoid Reflective Surfaces

🔹 Remove or cover any shiny or reflective materials near the light source or sensor.

3. Measurement Angle

🔹 Hold or position the sensor perpendicular to the emitting surface of the light source.

4. Measurement Distance

🔹 Keep within a 10–50 cm range for general light source characterization.

🔹 The “10–50 cm” distance is a commonly used practical guideline in lighting measurement practice, rather than a strict requirement defined by formal standards.

🔹 For photobiological safety (Blue Light Hazard Ranking, RG), follow IEC/EN 62471: For General Lighting Service (GLS) lamps, evaluate safety at the distance where the source emits 500 lux, and no closer than 200 mm.

🔹 Always maintain a consistent, fixed distance for repeatability.

🔹 Ensure the sensor’s field of view fully captures the emitting area.

🔹 Avoid shadows, obstructions, or ambient stray light during measurements.

2) Ambient / Task Lighting

This refers to general lighting in a space or light provided for a specific task (like reading or working).

Key Metrics to measure include:

🔹 Illuminance (lux)

🔹 Correlated Color Temperature (CCT)

🔹 Color Rendering Metrics – CRI (Ra and Re), TM-30 (Rf and Rg)

🔹 Flicker – Flicker frequency, flicker percentage, flicker index, and Stroboscopic Visibility Measure (SVM)

Room Condition Requirements

🔹 Ensure normal occupancy conditions (e.g., curtains/blinds as usual)

🔹 Horizontal Surface (Desk/Workplane): Height ≈ 0.8 m (typical tabletop)

🔹 Vertical Surface (Whiteboard/Wall Art): Position sensor perpendicular to the wall

🔹 Take multiple readings and average the results for accuracy

Based on the WELL Building Standard v2 – Feature L02: Visual Lighting Design

To meet illuminance thresholds, the project must refer to one of the following recognized standards:

🔹 IES Lighting Library – Lighting Applications Standards Collection

🔹 EN 12464-1:2021 or EN 12464-2:2014

🔹 ISO 8995-1:2002(E) (CIE S 008/E:2001)

🔹 GB50034-2013

🔹 CIBSE SLL Code for Lighting

WELL Performance Verification Conditions

To ensure compliance, testing must be conducted under these controlled conditions:

🔹 Nighttime measurements only – to eliminate daylight interference

🔹 Supplemental lighting, if present, must be turned on and positioned on the work surface in typical usage configuration

🔹 Each sample must be taken on the horizontal plane, with the device aperture facing upward

⚠️ Important: Ensure the shadow of the human does not fall on the device aperture during measurement

Tips: Use In. Licht Ultra’s PDF Report for WELL Submission

In. Licht Ultra automatically generates detailed PDF reports that include all required measurement data. These PDF reports can be used directly as supporting documentation for WELL Performance Verification under Feature L02 and other light-related criteria.

3) Human-Centric Lighting

Key Human-Centric Metrics to measure:

🔹 Circadian Action Factor (CAF)

🔹 Equivalent Melanopic Lux (EML)

🔹 Melanopic Equivalent Daylight Illuminance (M-EDI)

🔹 Scotopic/Photopic Ratio (S/P Ratio)

🔹 Circadian Stimulus (CS)

🔹 Blue Light Hazard Ranking (RG)

Based on WELL Building Standard v2 – Feature L03: Circadian Lighting Design

For Workstations Used During the Daytime

Electric lighting must achieve the following thresholds for at least 4 hours (beginning no later than noon) at a height of 18 in (≈ 45 cm) above the work plane in regularly occupied spaces

Standard thresholds:

🔹 ≥ 150 EML [136 M-EDI] (1 point)

🔹 ≥ 275 EML [250 M-EDI] (3 points)

For thresholds with enhanced daylight:

🔹 ≥ 120 EML [109 M-EDI] + L05 or L06 (1 point)

🔹 ≥ 180 EML [163 M-EDI] + L05 or L06 (3 points)

For Residential Units (Dwelling Spaces)

In addition to the thresholds mentioned above, the lighting must be dimmable. If automated, it should dim automatically after 8:00 p.m.

Measurements must be taken:

🔹In living rooms and kitchens at 55 in (≈ 140 cm) in the center of the room

🔹In studio apartments or guest rooms, also test in the center

🔹For workstations, at 18 in (≈ 45 cm) above the work plane

WELL Performance Verification Conditions

WELL v2 (and later):

Take samples at night using only electric light to eliminate daylight influence.

Sampling Position:

🔹Measure on the vertical plane at 45 cm (18 in) above the work plane, with the aperture facing the direction of gaze

🔹Light levels must be measured on the vertical plane at eye level to simulate the light entering the eyes

🔹Use a tripod and ensure a stable surface

🔹Avoid casting human shadows on the sensor

🔹Where no working planes are present , take 4 measurements in the center of the space at 140 cm (55 in) in orthogonal directions. The median value of the four measurements constitutes one sample point.

Reporting & Compliance

Report the illuminance levels in lux and the spectral power at 5 nm increments from 380 nm to 730 nm.

WELL Submission Tip

The CSV report generated by In. Licht Ultra includes all required WELL metrics and can be used for WELL Performance Verification.