Sintra yellow tram no.7 with simulated orthochromatic BW (left) and panchromatic BW (right) versions of the original colour photo.
Because we are used to panchromatic black & white these days, not everyone realises that old photos are often of a different kind of black & white. E.g. a tram on an old photo looking dark, could have been yellow.
The colours of visible light are determined by the wavelength. The spectrum goes from shorter to longer waves in the order violet – blue – green – yellow – red. With old photos often orthochromatic emulsions were used that are little sensitive to light of the longer wave lengths like red and yellow, but very sensitive to short wave lengths like blue and violet. Because of this red and yellow look very dark on many old photos while blue and violet turn very light.
Later often yellow or red filters were used to compensate for the difference in sensitivity for the colours. However that came at the cost of a longer exposure time causing motion blur and/or a larger diaphragm aperture causing less depth of field.
In later years panchromatic emulsions became common, that are more equal sensitive for all colours.
Below are a few example photos with both orthochromatis and panchromatic black and white simulated
This is an example photo of a Lisboa Remodelado with the classic yellow livery. At the left a blue tuk-tuk.This is the orthochromatic version of the example photo. The yellow of the tram turns very dark while the blue of the tuk-tuk is very light.This is the panchromatic B&W version of the example photo. Both the yellow of the tram and the blue of the tuk-tuk are shown as different shades of gray.This second example shows the Tramvia Blau in Barcelona.This is the orthochromatic B&W version of the Tramvia Blau. The blue tram turns light, but the yellow decorative stripes are dark. The clouds cannot be distinguished anymore from the blue sky.This is the panchromatic B&W version of the Tramvia Blau. The blue tram turns dark, but the yellow decorative stripes are light. The clouds can be distinguished from the blue sky.This third example shows a yellow tram with a green trailer in Porto.On the orthochromatic B&W version the yellow tram is slightly darker than the green trailer. Take note that the brownish road surface is a bit dark too, while the blue of the no-parking sign at the right turned white.The panchromatic version of this example shows the yellow tram as rather light, while the green trailer is dark.This is a detail of an old postcard. At first sight the trams look equal, but at better inspection the one at the right is darker than the other. While the lighter of the two has white fleet numbers, the darker has black fleet numbers. It’s known that the Porto trams were repainted from green to yellow with the fleet numbers from white to black in 1933. Conclusion is that this photo was taken in 1933 using an orthochromatic emulsion. The left tram is still green while the right one is already yellow.
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