Part 1: Proof of the hardness of light
From photon to room light
A photon has no mass, but it has energy and momentum, depending on its frequency. The luminous flux of a light source in lumens is weighted by the brightness sensitivity of the eye, which peaks at 555 nm (theoretically 683 lm/W). How ordered this luminous flux is when emitted into the room is determined first by the optical system of the luminaire – a diffuse cover or a clear lens. This primary light modulation shapes the character of the room light. Secondary light control through reflections only makes it softer: a focused beam breaks up into many small points of light on a larger surface, and the light arrives from a larger solid angle.
Measuring space and influencing factors
The upper hemisphere is considered; for measurement it is reduced to a half-space of 360° × 180° with 64,800 measuring points, horizontal or vertical depending on the reference surface. The measuring head captures it in 180 steps of 1° each, because the axis of rotation lies centrally above the shadow caster. Influencing factors are:
- the light spread factor (LwF) of the luminaire,
- the number and position of the light sources,
- the distance, location and orientation of the measuring point,
- the measuring apparatus, shadow caster, sensor surface and evaluation software,
- theoretically the measuring time, because of increasing interreflections.
Distance D as the measure
From the 180 measuring steps, the measurement with the highest HgF – that is, the shortest distance D – is stored as the leading value. Until now, including in the master's thesis, the steepest gradient in the transition to the core shadow was taken as the measure. That does not reflect practice: when dimming, light hardness stays the same, but the gradient changes. What matters is therefore the distance D, the sum of the partial distances d. A short D means a high HgF, a long D a low one.
Part 2: Master's thesis “Reproducibility of lighting situations using the hardness of light”
Author: Clarissa Borgstädt, M. A. – first examiner: Prof. Dr. Roland Greule, second examiner: Prof. Dr. Robert Mores (HAW Hamburg), topic and review: Dipl.-Ing. (FH) Markus Felsch.
Demonstrator
The basis was the LMK mobile air luminance camera from the lighting laboratory of HAW Hamburg. A lens extension with an internal diffuser film as projection surface, lined with black molton against stray light, ensures a fixed focus. A rotating shadow caster above the measuring surface produces the measurement images, which are analysed with an image evaluation program, LMK LabSoft and a spreadsheet evaluation. Tests with a lighting simulation program complemented the measurements.
Result and potential developments
The method proved that the hardness of light can be measured. Conceivable features of a production device are automated triggering of the 180 images or a video recording, a microcontroller that finds the decisive image and displays the HgF directly, wireless data transfer and position detection to output the values of several points as a spatial graphic.
Scale and design parameter
A scale from 0 to 100 suggests itself. Further measurements must show whether it should be linear or – in line with brightness perception – logarithmic. There cannot be uniform target values: offices, shops, restaurants and hotel lobbies have different requirements. Target values require an empirical evaluation with test subjects.
LiTG publication no. 35 describes modelling as the balance between diffuse and directed light, with a ratio of cylindrical to horizontal illuminance between 0.30 and 0.60 as an indicator. There are no normative specifications for cast shadows. Quantitative design values for the character of light are therefore missing. Introducing the HgF as an additional design parameter makes sense – for example for a high-contrast product presentation with a high HgF next to a sheltered customer zone with a low HgF.
