Remember this figure for the eclipse that will cross the Iberian Peninsula on 12 August. Keeping that date in mind, the ISO number (International Organization for Standardization) of the special glasses that must be used to view this once-in-a-century phenomenon safely should be easy to remember: 12312-2015.
Without them, ultraviolet radiation will damage the eye’s photoreceptors (cones and rods), while infrared energy will produce a direct thermal effect on the retina, causing solar retinopathy. Symptoms of this condition, which may take time to appear, can include blurred vision, spots or scotomas at the point of greatest visual acuity, metamorphopsia (distorted perception of shapes) and reduced tolerance to light, or photophobia, which can lead to headaches.
The total phase of the eclipse will not last more than 1 minute and 48 seconds - the closer you are to the line of totality, the more seconds you will be able to enjoy the event - anywhere along its path across Spain. It is in that brief moment that lucky observers may remove their approved glasses.
ISO standard 12312-2 applies only to certain products designed for direct observation of the Sun, or what astronomers call the photosphere: the Sun’s bright face. These products ensure that no more than 0.00032% of visible sunlight is allowed through. The Ministry of Science explains that "you must put the glasses on before looking at the Sun, look briefly for a few seconds and then lower your gaze, repeating observation of the eclipse always with pauses". You should not take them off before you stop looking at the Sun.
