Media College (Chương 2 : Ánh sáng) (1) Print
Written by tuyenphuc   
Saturday, 20 March 2010 10:19



Chương 2: Ánh sáng.

Cũng như Chương 1 là Âm thanh, Chương 2 của Media College nói về phần kỹ thuật ánh sáng. Nhưng vì là Media nên bài viết thiên về ánh sáng kỹ thuật của thu hình (video), không phải Stage lighting. Tuy vậy, về phần cơ bản, 2 loại hình cũng giống nhau, đáng để tham khảo. (tuyenphuc).


 01 par56-160

 

Phần 1: Thuật ngữ ánh sáng.

LightingTerminology.

Common Lighting Terminology

Ambient Light

The light already present in a scene, before any additional lighting is added.


Ambient light means the light that is already present in a scene, before any additional lighting is added. It usually refers to natural light, either outdoors or coming through windows etc. It can also mean artificial lights such as normal room lights.

Ambient light can be the photographer's friend and/or enemy. Clearly ambient light is important in photography and video work, as most shots rely largely or wholly on ambient lighting.

Unfortunately ambient light can be a real nuisance if it conflicts with what the photographer wants to achieve. For example, ambient light may be the wrong color temperature, intensity or direction for the desired effect. In this case the photographer may choose to block out the ambient light completely and replace it with artificial light. Of course this isn't always practical and sometime compromises must be made.

On the other hand, many of history's greatest photographs and film shots have relied on interesting ambient light. Unusual lighting can turn an otherwise ordinary shot into something very powerful.

Incident Light

Light seen directly from a light source (lamp, sun, etc).

Reflected Light

Light seen after having bounced off a surface.

Colour Temperature

A standard of measuring the characteristics of light, measured in kelvins.

Colour Temperature Chart

Colour temperature is a standard method of describing colours for use in a range of situations and with different equipment. Colour temperatures are normally expressed in units called kelvins (K). Note that the term degrees kelvin is often used but is not technically correct (see below).

02colour-temperature

Technically speaking...

Colour temperature means the temperature of an ideal black body radiator at which the colour of the light source and the black body are identical. (A black body is a theoretical radiator and absorber of energy at all electromagnetic wavelengths.)

Colour Temperature in Video

For video operations the relevant temperatures range from around 2,000K to 8,000K — these are common lighting conditions. In practical terms this usually means selecting lights, gels and filters which are most appropriate to the prevailing light or to create a particular colour effect. For example, a camera operator will select a "5600K filter" to use outside in the middle of a sunny day.

Terminology

  • When referring to the unit kelvin, it is not capitalised unless it is the first word of a sentence. The plural is kelvins (e.g. "The light source is approximately 3200 kelvins").
  • The symbol is a capital K (e.g. "The light source is approximately 3200K").
  • When referring to the Kelvin scale, it is capitalised (e.g. "The Kelvin scale is named after William Thomson (1824 – 1907), also known as Lord Kelvin".

Degrees kelvin

According to the The International System of Units (SI) , colour temperatures are stated in kelvins, not in degrees Kelvin. The "degrees" part of the name was made obsolete in 1967. However, the "degrees" reference has remained in common use in media industries.

Contrast Ratio

The difference in brightness between the brightest white and the darkest black within an image.


Contrast Ratio is a measurement of the difference in brightness between the whitest white and the darkest black within an image. A ratio of 300:1 means the brightest point in the image is 300 times as bright as the darkest point. A higher contrast ratio therefore means a larger difference in brightness.

Contrast ratio is of interest in two situations:

  1. Cameras: When recording an image (video, film, photography)

In video and film work it is important to understand what sort of contrast ratio your camera is able to reproduce. A high contrast ratio means that brighter and darker areas of the image will be recorded with more accuracy and apparent detail.

Most people don't need to know the actual specifications but video makers need to be aware that video has a relatively low contrast ratio. If an image includes extreme light and dark, the camera will struggle to reproduce both. Bright areas will appear over-exposed and dark areas will appear "crushed" (all black, lacking detail).

The example on the right illustrates a common problem for sports coverage in stadiums — the difference between the sunlit areas and the shadows is significant.

The best way to minimize problems with contrast ratio is to avoid having very bright and very dark objects in frame at the same time. When dealing with human subjects it makes sense to avoid white and black clothing. Also, be wary of caps and sunglasses in strong light — they can create terrible over-contrast on the face.

If you can't alter the framing, either add lighting to the dark areas or filter the bright areas.

When all else fails, the standard approach is to expose for the subject. If some parts of the picture are too bright or too dark it's not the end of the world — people are used to seeing this.

Note: Film cameras generally perform better than video, but still do not reproduce the same range that the human eye would see.

02A stadium

  1. TVs, Monitors, etc. When choosing or setting up a playback device (TV, computer monitor, etc)

Contrast Ratio is a specification given with most good televisions and monitors. It refers to the device's ability to reproduce different levels of brightness.

A higher contrast ratio is more desirable — 500:1 is quite good.

Unfortunately the contrast ratio specification given by TV manufacturers has become somewhat abused. There is a limit beyond which this specification loses significance, as further improvements aren't noticeable in the real world. Do not be too concerned with figures higher than 2000:1, and figures over 10,000:1 don't have much relevance at all.

Note: Monitors also vary in their ability to display levels of gray, i.e. their grayscale performance. This can also effect the detail in darker areas of the image.

Key Light

The main light on the subject, providing most of the illumination and contrast.

Fill Light

A light placed to the side of the subject to fill out shadows and balance the key light.

Back Light

A light placed at the rear of a subject to light from behind.
Light directly from a source such as the sun, traveling undisturbed onto the subject being lit.

The Standard 3-Point Lighting Technique

The Three Point Lighting Technique is a standard method used in visual media such as video, film, still photography and computer-generated imagery. It is a simple but versatile system which forms the basis of most lighting. Once you understand three point lighting you are well on the way to understanding all lighting.

The technique uses three lights called the key light, fill light and back light. Naturally you will need three lights to utilise the technique fully, but the principles are still important even if you only use one or two lights. As a rule:

  • If you only have one light, it becomes the key.

If you have 2 lights, one is the key and the other is either the fill or the backlight


Key Light

This is the main light. It is usually the strongest and has the most influence on the look of the scene. It is placed to one side of the camera/subject so that this side is well lit and the other side has some shadow.

03 lighting-floorplan-key


Fill Light

This is the secondary light and is placed on the opposite side of the key light. It is used to fill the shadows created by the key. The fill will usually be softer and less bright than the key. To acheive this, you could move the light further away or use some spun. You might also want to set the fill light to more of a flood than the key.

04 lighting-floorplan-fill


Back Light

The back light is placed behind the subject and lights it from the rear. Rather than providing direct lighting (like the key and fill), its purpose is to provide definition and subtle highlights around the subject's outlines. This helps separate the subject from the background and provide a three-dimensional look.

05 lighting-floorplan-back

If you have a fourth light, you could use it to light the background of the entire scene.


Soft Light

Light which appears to "wrap around" the subject to some degree. Produces less shadows or softer shadows.

Spot

A controlled, narrowly-focused beam of light.

Flood

A broad beam of light, less directional and intense than a spot.

Tungsten

Light from an ordinary light bulb containing a thin coiled tungsten wire that becomes incandescent (emits light) when an electric current is passed along it. Tungsten colour temperature is around 2800K to 3400K. Also known as incandescent light.

Halogen

Type of lamp in which a tungsten filament is sealed in a clear capsule filled with a halogen gas.

Fresnel

A light which has a lens with raised circular ridges on its outer surface. The fresnel lens is used to focus the light beam. Incandescent Incandescent lamps produce heat by heating a wire filament until it glows. The glow is caused by the filament's resistance to the current and is called incandescence.

 

Measuring Light Unit.

There are many different units for measuring light and it can get very complicated. Here are a few common measurement terms:

Candela (cd)
Unit of luminous intensity of a light source in a specific direction. Also called candle.
Technically, the radiation intensity in a perpendicular direction of a surface of 1/600000 square metre of a black body at the temperature of solidification platinum under a pressure of 101,325 newtons per square metre.

Footcandle (fc or ftc)
Unit of light intensity, measured in lumens per square foot. The brightness of one candle at a distance of one foot. Approximately 10.7639 lux.

Lumen (lm)
Unit of light flow or luminous flux. The output of artificial lights can be measured in lumens.

Lux (lx)
Unit of illumination equal to one lumen per square metre. The metric equivalent of foot-candles (one lux equals 0.0929 footcandles). Also called metre-candle.

These are some common types of light you'll often hear about in film, video and photography. Note that these definitions are not always rigid and some people may interpret them a little differently.

Blonde

1000-2000w, used as a key flood light for large areas.

 06 ianiro-blonde-2kw

A blonde light is basically a bigger version of a redhead. Power rating can be 1000 to 2000 watts, although the term generally refers to a 2000w open-face unit.

These are powerful lights, useful as key floodlights for lighting large areas.

The example pictured here is an Ianiro 2Kw blonde.

Note: The term "blonde" is often used loosely — there is no rigid definition.

Redhead

650-1000w, used as a key flood light for large areas.

07 arrilite-800-180 

Red Head is a term used to describe general-purpose lights common in visual media work. Power rating is around 650-1000w, typically 800w.

Red heads can be used as a key flood light for large areas, but are also useful as fill and backlights.

The example pictured here is an Arrilite 800w open-face focusing tungsten floodlight. The beam is focused using the yellow control at the back — this adjusts the reflector rather than the lamp, which should give the lamp a longer life because it is not being moved.

Note: The term "redhead" is often used loosely — there is no rigid definition.

Pepper Light

100-1000w, small light used as a more focused key or fill light.

08 ltm-pepper-200-180 

Pepper lights are small lights, around 100 - 1000w (200w is common). They can be used as key or fill lights in small areas, or to light a certain feature, create lighting contrast and effects, etc.

Pepper lights can be also be used to accent a person's eyes.

HMI

A high-quality type of light which uses an arc lamp instead of filament bulb.

 09 hmi-1200

HMI (Hydrargyrum Medium-Arc Iodide) is a type of light which uses an arc lamp instead of an incandescent bulb to produce light.

HMI lights are high-quality and correspondingly expensive. They are popular with film and television production companies but their price puts them out of reach of those with modest budgets.

HMI lights require a ballast, an electronic (or magnetic) device which provides the ignition pulse and regulates the arc.

 

Advantages of HMI lights include:

  • Efficiency: 2 to 5 times as efficient as incandescents. This means they use less power and run cooler.
  • Colour temperature: HMIs run at around 5600K, daylight temperature. This makes daylight shooting easier, as well as eliminating loss of light from gels (which are necessary with incandescents).
  • Light Quality: Directors of photography speak highly of the light produced by HMIs.

Halogen Work Lamp

150-500w, used as a key flood light for lighting large areas. This is a low-budget lighting solution.

10 halogen-work-lamp-small 

Halogen work lamps are designed for workplaces and similar situations where a flexible means of providing strong lighting is required, e.g. builders, mechanics, etc. Work lamps come in various sizes and configurations, from portable units with handles to stand-mounted multi-head versions. Power rating is generally 150w to 500w.

Halogen work lamps are sold by trade suppliers and DIY stores.

11 halogen-work-lamp 

Work lamps can be a cheap option for video lighting. They are useful as a key flood light for lighting fairly large areas.

You must remember though, that these lamps are not specifically designed for video or photography. They are not ideal and usually require a diffusion gel for the best effect. The protective grills may also interfere with the light (you may be able to remove them).

You might also want to consider waterproof work lamps for extra safety, especially if you are working outdoors.

Other Lights

Domestic light bulbs can be used at a pinch, ideally as a secondary light such as fill or backlight.
Many video cameras have built-in lights or the ability to mount a light — these are useful in emergencies but provide poor quality lighting.

Chinese Lanterns

A low-cost light, useful in some situations.

12 chinese-lantern-01 

chinese lantern is a paper ball (or cylinder, cube, etc) with a light bulb in the middle. They are very cheap to buy and use a standard household light bulb.

For video and photographic work, chinese lanterns are obviously limited by the small amount of wattage. However they do create a nice light that can be useful for close-up shots. In a wider setting, multiple lanterns can be used for effect.

Being made of paper, chinese lanterns are potentially dangerous. They can catch fire so be careful. For storage they can be collapsed to a disk the same diameter as the expanded sphere.

Instruments / Housing

Fresnel

A light which has a lens with raised circular ridges on its outer surface which are used to focus the light beam.

13 fresnel-30     14 fresnel-200

chinese lantern is a paper ball (or cylinder, cube, etc) with a light bulb in the middle. They are very cheap to buy and use a standard household light bulb.

For video and photographic work, chinese lanterns are obviously limited by the small amount of wattage. However they do create a nice light that can be useful for close-up shots. In a wider setting, multiple lanterns can be used for effect.

Being made of paper, chinese lanterns are potentially dangerous. They can catch fire so be careful. For storage they can be collapsed to a disk the same diameter as the expanded sphere.

 

 Xem tiếp Phần 2: Thiết bị ánh sáng. Lighting equipment.