Pierre - Jan 2020
Automated solar reflectors (heliostats) are not a recent invention. The term "heliostat" was first coined by the Dutch physicist Gravesande in 1742 (helio for sun, stat for static). The first heliostats thus date from the 18th century, before the generalization of electric lamps. They were used to obtain a fixed solar ray to make observations with the microscope or other physics experiments such as the measurement of the speed of light (fr).
In the absence of electronics, the early inventors (Silbermann, Foucault, Ekling...) exploited the fact that for a given day, the apparent path of the sun is included in a single plane. Heliostats had only one axis of rotation, which was driven by a spring-loaded clock system and had to be parallel to the Earth's axis of rotation. The inclination of this axis was usually manually adjustable according to the latitude. A second setting allowed adaptation to the seasons.
Once this is understood, it is easy to reflect the sun's rays towards the celestial North Pole. To reflect them to a chosen point, the various inventors have used several strategies. The most intuitive method is to add a second free mirror, as illustrated by this DIY. However, early inventors preferred to use different systems to constrain a single mirror to the desired direction of reflection. Some heliostats such as Prazmowski's were designed to reflect the sun's rays in a limited plane. A second mirror was then necessary.
For the little anecdote, the Silbermann heliostat was designed by Mr. Soleil (the King's optician) and its value was probably close to a year's salary for a worker. Prazmowski's heliostat was half as expensive. Today, collectors exchange these machines between 1000 and 10000 €.
Nowadays, electronics make it possible to build heliostats with two axes of rotation, which makes it possible to automate over the whole year, without having to impose the positioning of the installation. This is the case for the Helioreflect domestic heliostat.
Older inventions to improve daylighting also include prismatic glazing, which was marketed in the USA and Europe from the end of the 19th century onwards. In particular, they were used in clothing stores, where only natural light allow to distinguish the true colour of clothes.
In France, the société continentale des Verres Soleil pushed the concept to the point of creating light reflectors arranged as awnings above the windows. Today, prismatic glass is found in some glass bricks. The concept is also marketed in the form of films to be glued on windows.
Image Credits: Sun path, Silbermann Heliostat, Luxfer advert.
Other sources (fr): Sur la théorie des héliostats (Bulletin Astronomique, 1884), Héliostat de Silbermann, Maison Soleil, Description détaillée de l'héliostat de Prazmowski (1877), Catalogue héliostat de Prazmowski (1904), Valeur du Franc de l'époque, Société continentale des Verres Soleil.