Rather recently Vishay announced a miniature sensor under the name VEML6075 with two channels centred at . It is not just a sensor but it has all the electronics for temperature compensation and for converting the analogue signal from the sensor into digital data with a rough calibration applied. The package of this sensor is 1 mm thick and 2 mm times 1.25 mm in area. The sensor itself is much smaller and it follows reasonably well the cosine law without any diffuser. Price? Less than 2€… and between 4 and 7 € for a breakout board.
I haven’t tested it yet as a sensor. However, today I was testing a macro lens and I took a pair of photographs at rather high magnification, and as it fits a UV sensor, I photographed it both in visible and in UV-A radiation.
The whole image is 3 mm tall by 4 mm wide. It is slightly cropped from the full frame. Light source Convoy 2+ 365nm UV-A flashlight filtered with a visible blocking filter. Filter on objective zwb1 2mm thick, objective Zuiko 38mm f:3.5 macro (Olympus OM-System ca. 1973-1975+, single coated early version).