Q2Utilization Of Electrical Power

Photometry and the Photovoltaic Method

Photometry is the branch of measurement science concerned with quantifying visible light in terms of its perceived brightness to the human eye, encompassing the measurement of luminous flux, luminous intensity, illuminance, and luminance of light sources and illuminated surfaces. Photometric measurement methods are broadly classified into visual methods, which rely on a human observer's judgement of when two adjacent illuminated surfaces (a test surface and a reference/standard surface) appear equally bright (photometric balance), and physical (photoelectric) methods, which use a light-sensitive electrical device to convert incident light directly into a measurable electrical signal proportional to illuminance, eliminating the subjectivity and fatigue limitations inherent in visual observation.

The photovoltaic method of photometry uses a photovoltaic cell (such as a selenium or silicon photocell) that generates a small electric current directly proportional to the intensity of light falling upon its light-sensitive surface, without requiring any external bias supply, this current being measured by a sensitive microammeter that can be directly calibrated in lux (illuminance units) once the cell's response has been characterized against a known reference light source. To measure the illuminance at a given point, the photovoltaic cell is simply placed at that point, oriented appropriately (usually facing the direction from which illuminance is to be measured), and the meter reading is taken directly, making the photovoltaic method extremely quick, convenient, and objective (free of any observer judgement) compared to visual photometric balance methods.

Limitations of the Photovoltaic Method

• Spectral response mismatch: the photovoltaic cell's sensitivity to different wavelengths of light does not exactly match the human eye's photopic (daylight) spectral sensitivity curve, so readings can be systematically inaccurate for light sources with a spectral distribution significantly different from that used to originally calibrate the cell, unless an appropriate colour-correction filter is fitted over the cell.
• Fatigue and ageing effects: the sensitivity of some photovoltaic cell materials can drift or temporarily fatigue under prolonged or intense illumination, and can degrade with cell age, requiring periodic recalibration against a known standard source for continued accuracy.
• Cosine response error: an ideal photometer should respond to obliquely incident light in accordance with Lambert's cosine law, but the physical construction of practical photocells (recessed sensing surface, cover glass reflections) can cause deviations from true cosine response at large angles of incidence, introducing measurement error for light arriving from oblique directions.
• Non-linearity at very high or very low illuminance levels: the current output of some photovoltaic cell types is not perfectly linear with illuminance across their full operating range, requiring careful calibration or the use of measurement ranges where linearity is assured.
• Temperature sensitivity: the output of some photocell materials varies somewhat with ambient temperature, introducing a further potential source of measurement error if not compensated for.

Street Lighting

Street lighting is designed primarily to provide adequate, reasonably uniform illuminance along a roadway to ensure the safety of vehicular and pedestrian traffic during hours of darkness, with the specific design illuminance level, uniformity ratio, and glare control requirements depending on the road classification (main highway, arterial road, or residential street) and expected traffic density. Street lighting design must balance the mounting height and spacing of lamp poles against the luminous intensity distribution of the chosen luminaire to achieve the specified average illuminance with acceptable uniformity (avoiding excessively bright pools directly beneath each pole with dark gaps between poles), while also controlling glare toward oncoming drivers, historically using high-pressure sodium or mercury vapour lamps and increasingly LED luminaires in modern installations for their superior efficacy, controllability, and long life.

Flood Lighting

Flood lighting, in contrast to street lighting's roadway-uniformity objective, is the deliberate illumination of a defined outdoor area or object (building facades, monuments, stadiums, construction sites, storage yards) with a controlled, high-intensity beam projected from one or more floodlight luminaires with reflectors designed to concentrate and direct the light output into the specific beam angle required to cover the target area from the available mounting positions, and is used for architectural display, sports and event lighting, and general area/security lighting where the priority is adequate area coverage and, in decorative applications, aesthetic effect, rather than the strict roadway uniformity and glare-control criteria central to street lighting design.

Beyond the photovoltaic method described above, illuminance and luminous intensity can also be measured using photoemissive and photoconductive cells, which respond to incident light by, respectively, emitting electrons from a photosensitive cathode under an applied bias voltage, or by exhibiting a change in electrical resistance proportional to illuminance; both alternative technologies require an external power supply to operate (unlike the self-generating photovoltaic cell) but can offer different sensitivity ranges and spectral responses suited to particular specialized photometric applications, such as very low light level measurement, where the photovoltaic cell's inherently self-generated, unamplified signal may become too small to measure reliably without additional amplification.

In practical lighting survey work, portable photovoltaic lux meters remain by far the most common instrument used by lighting engineers for routine illuminance verification of street lighting, factory, and office installations, precisely because of their combination of adequate accuracy (once appropriately colour- and cosine-corrected), ruggedness, low cost, and ease of use in the field compared to the more elaborate laboratory-grade visual photometers historically used for precision photometric standardisation work.

A well-designed street lighting scheme is typically specified in terms of three related criteria rather than average illuminance alone: the average luminance (or illuminance) of the road surface, the overall uniformity ratio (minimum to average illuminance, ensuring no unacceptably dark patches occur between poles), and the glare rating of the installation as experienced by an approaching driver; achieving all three criteria simultaneously requires the lighting designer to jointly select luminaire mounting height, pole spacing, arm length/overhang, and the specific luminous intensity distribution (light distribution curve) of the chosen luminaire, rather than any single parameter in isolation, which is why standard street lighting design practice in most national codes specifies minimum values for all three criteria according to the classification of the road being illuminated.

This closes the requested discussion of photometry, the photovoltaic method and its limitations, together with the short notes on street lighting and flood lighting.