Glare

What is glare and why does it matter in a house?

Glare is excessive brightness or strong luminance contrast in the field of view that makes a space uncomfortable or harder to use. In a house it usually comes from direct sun through a large window, from reflections off glossy surfaces, or from poorly placed electric lights. It often drives the choice of a tieniace zariadenie (solar shading device) more than any energy target does.

Glare matters because it changes behaviour. Occupants who cannot read a screen without squinting close the blinds, and the room then loses its daylight.

What is the difference between discomfort glare and disability glare?

Discomfort glare is the annoyance or fatigue caused by a bright source in the field of view, even when you can still see clearly. Disability glare is more serious: the bright light scatters inside the eye and reduces the ability to see objects, such as a pale document in front of a bright window.

In a house, discomfort glare is the everyday problem. Designers quantify it with metrics such as the Daylight Glare Probability, but a homeowner usually needs the principle more than the calculation.

How does window orientation change the glare problem?

Orientation decides when glare happens. East and west facades are the hardest, because the low morning and afternoon sun shines almost straight into the room, where an overhang cannot block it. South facades receive a high summer sun that an overhang can cut off, while the lower winter sun rarely reaches the eyes of someone seated away from the glass. North facades give soft, diffuse light with little glare.

The trade-offs are summarised in the daylighting design article. Place long-stay spaces such as desks away from the low-sun direction, and use the low-sun facade for rooms that are used briefly.

Which external shading measures reduce glare best?

External shading cuts the brightness at the window before the light reaches the room. The main options are:

• A fixed horizontal overhang over south-facing glazing, which blocks high summer sun.
• Vertical fins or an adjustable slnolam (brise-soleil) on east and west facades.
• External screens or roller shutters, lowered when the sun shines directly into the room.
• Trees, climbers, or pergolas, which need no moving parts but offer little control in winter.

For glare, the key point is that external measures act before the glass, so the room stays calmer even with the window open.

How do internal blinds and curtains help, and where do they fall short?

Internal blinds and curtains are the most flexible tools an occupant has. Adjustable slats can block direct sun while keeping the view and some daylight.

They fall short in two ways. An internal blind absorbs solar radiation after it has passed the glass, so it helps with glare far more than with heat. Light also leaks around slats, cord openings, and side gaps, so a blind that looks closed may still let a bright strip through.

How should luminaires be placed to avoid glare?

Electric lighting causes glare when a bright source sits inside the field of view, such as a bare lamp at eye level, a downlight directly above a screen, or a fitting behind a seated person whose reflection shows on the monitor. The fix is mostly placement and shielding, not wattage.

• Keep fittings out of the direct line of sight from normal seated and standing positions.
• Use shielded or diffused fittings, so the source is not a bare bright point.
• Light the workspace with indirect or wall-washing light from the ceiling.
• Link dimming to daylight levels, so electric light matches the daylight available. A DALI system can do this.

Do matte finishes reduce reflected glare?

Yes. Reflected glare comes from specular surfaces such as polished stone worktops, glossy tiles, high-gloss cabinet doors, and metallic fittings, which send a bright image of the window or lamp toward the eye. Matte or satin finishes scatter the light instead, so the same window produces a soft reflection rather than a hard bright patch.

Finish also affects contrast. A dark glossy surface next to a bright window creates the strongest contrast, while a matte surface keeps the room visually calm, whatever its tone.

Source	Typical situation	Main kind of glare	First remedy
Low sun through large glazing	Sitting with a west or east window in the line of sight	Discomfort, sometimes disability	External shading or moving the desk
Glossy floor or worktop	Reflection of the window toward the eye	Discomfort	Matte or satin finish
Bare lamp or downlight over a screen	Light source inside the field of view	Discomfort	Reposition and shield the fitting
Screen placed in front of a window	Backlit screen against a bright background	Discomfort, reduced visibility	Turn the desk so the window is at the side

How is glare different from overheating risk and the daylight factor?

Overheating risk and the daylight factor are different problems. Glare is a visual effect caused by too much brightness or contrast. Overheating is a thermal effect caused by too much stored solar heat. The daylight factor measures how much daylight reaches a point inside the room compared with the outside, so it shows whether a room is bright enough, not whether it is too bright.

Remedies overlap. External shading reduces both glare and overheating, while a larger window raises the daylight factor but can increase both problems. Each should be checked separately.

Measure	Acts on	Strength against glare	Trade-off
Orientation and room layout	Position of the low sun	Prevents glare from the start	Fixed once the design is set
External shading	Brightness at the window	Strong, acts before the glass	Cost, weather exposure, maintenance
Internal blinds and curtains	Brightness inside the room	Good control and flexibility	Leaks light, adds little against heat
Matte or satin finishes	Reflected light	Removes specular reflections	Applies to surfaces, not the window

How is glare checked before a house is built?

Designers check glare with daylight simulation, which calculates a glare metric from the window geometry, surface reflectances, and the sun path for the site. Because the result depends on the viewer's position, check the seats that matter most, such as the desk, the sofa, and the bed.

The European standard EN 17037 treats glare protection as one of four daylight aspects, alongside daylight provision, view out, and sunlight exposure. Its criteria should be read from the current text of the standard rather than from rules of thumb.