Video wherever the decision to open the door depends on knowing who is there - which in practice is most residential and many commercial entrances.

An audio station confirms that someone is there and lets a conversation happen. It is simple, cheap, robust and entirely adequate where the caller is expected or where the door leads somewhere unimportant.

A video station changes the nature of the decision. The resident sees who is at the door before releasing it, which matters for deliveries, for unexpected callers and for anybody who does not want to open a door on the basis of a voice.

It also provides a record where the system supports capturing an image of each call, which is useful after the event.

The cost difference has narrowed to the point where video is the default for new work. Where audio is chosen it is usually for a specific reason - an existing wiring system that cannot carry video, or a position where a camera raises privacy objections.

Ingress protection matched to the exposure, and impact resistance matched to how public the position is - and both are worth over-specifying because replacement is disruptive.

A station under a substantial porch is far less exposed than one on an open wall facing prevailing weather. Ratings that resist driving rain cover most positions; higher ratings belong where the panel is fully exposed or subject to washing down.

Water rarely enters through the front. It enters at the cable entry, at a poorly sealed flush mounting, or by running down the wall and behind the panel - which is why the mounting detail is as decisive as the panel's own rating.

Impact resistance matters at street-facing entrances, in public buildings and in student and social housing. Stainless or aluminium panels with recessed buttons and toughened lens covers survive where plastic does not.

Vandal-resistant fixings are worth specifying at the same time. A panel that cannot be removed without tools is one that stays where it was fitted.

With wide dynamic range and appropriate illumination - because the two hardest cases, backlight and darkness, are both extremely common at entrances.

A visitor standing with bright sky behind them is rendered as a silhouette by a camera without wide dynamic range. That is precisely the situation at most doorways during daylight, and it defeats the purpose of having video at all.

At night, the camera needs its own illumination. Infrared gives a monochrome image and works well; white light gives colour and is more intrusive. Either way it must illuminate the face without overexposing it - a bright illuminator close to the subject produces a washed-out face against a black background.

Positioning is part of the answer. A station facing east or west will be looking into the sun at some point each day, and orientation should be considered when the position is chosen.

A wide angle lens matters too, since visitors stand closer than expected and a narrow field shows a forehead.

Mounting heights and reach ranges that suit seated and standing users, controls that can be found and operated without fine vision, and an alternative to a purely audible interaction - with the specific figures set by the local standard.

Requirements differ substantially between jurisdictions, so the governing document is the local accessibility standard or building regulation, not a general rule.

The principles are consistent. Controls should be within reach of a person using a wheelchair and usable by someone standing, which usually defines a band rather than a single height. Buttons should be tactile and visually contrasting. Where a name directory is used, text must be legible and illuminated.

For visitors who cannot hear, a visual indication that the call has been answered and that the door has been released is essential - a purely audible confirmation excludes them.

Induction loop provision at the panel assists hearing aid users and is required in many public buildings.

Establish the applicable standard before selecting the panel, since it constrains both the product and the mounting.

Flush for appearance and vandal resistance, surface for retrofit and for walls that cannot be cut - and the weatherproofing detail differs between them.

A flush-mounted panel sits in a back box recessed into the wall, presenting a clean face with little to grip. It looks better and resists interference, and it is the usual choice for new build.

Surface mounting needs no chasing, which makes it the practical answer for retrofit, for solid masonry, for listed structures and for gate posts.

Weatherproofing differs. A flush installation depends on the back box being sealed and drained, because water running down the wall face reaches the recess. A surface installation needs a rain hood and a sealed cable entry from behind or below.

Both need the cable entry to come from below or through the wall rather than from above, and both benefit from a hood in exposed positions.

Depth is the constraint people miss: flush boxes for modular panels are deep, and a cavity or a thin partition may not accommodate them.

It depends entirely on the system - two-wire, structured cabling for IP, or a multi-core bus - and the choice is effectively made when the system is selected.

Two-wire systems carry power, audio, video and control over a single pair, which makes them the standard answer for retrofit in existing buildings where only a limited cable exists. Distances and video quality are constrained but usually adequate.

IP systems use ordinary structured cabling with PoE, which suits new build and buildings with existing data cabling. They integrate with networks, apps and access control, and scale well.

Proprietary bus systems sit between the two and are common in larger residential developments.

What matters practically is that the door station, the indoor units and the controller must belong to one system - these are not interoperable between manufacturers.

Where existing cable is being reused, test it before committing. A pair that carries audio may not carry video reliably over the distance involved.

Through the door release and, in integrated systems, through a shared controller - and the interface deserves attention because two systems releasing one door is a common source of confusion.

At its simplest, the door entry system operates a lock release and access control does the same independently. Both work, and the lock has two sources of release.

Better arrangements integrate: the entry panel's reader is part of the access control system, so credentials, permissions and audit are managed in one place and a lost card is revoked once.

IP door stations frequently integrate natively with access control and video management platforms.

The practical points are to decide which system holds the credential database, to ensure the emergency exit arrangements are unaffected by either - free egress must never depend on a working system - and to confirm the fire alarm interface releases the doors it is required to.

Those life-safety interfaces are governed by local regulation and should be confirmed with the fire strategy rather than assumed.